Astronomy
&Astrophysics

A&A 650, A205 (2021)
https://doi.org/10.1051/0004-6361/202140381
© ESO 2021

A search for transiting planets around hot subdwarfs

I. Methods and performance tests on light curves from Kepler, K2, TESS, and
CHEOPS?

V. Van Grootel1 , F. J. Pozuelos1,2, A. Thuillier1, S. Charpinet3, L. Delrez1,2,4, M. Beck4, A. Fortier5,6, S. Hoyer7,
S. G. Sousa8, B. N. Barlow9, N. Billot4, M. Dévora-Pajares10,11, R. H. Østensen12, Y. Alibert5, R. Alonso13,14,

G. Anglada Escudé15,16, J. Asquier17, D. Barrado18, S. C. C. Barros8,19, W. Baumjohann20, T. Beck5, A. Bekkelien4,
W. Benz5,6, X. Bonfils21, A. Brandeker22, C. Broeg5, G. Bruno23, T. Bárczy24, J. Cabrera25, A. C. Cameron26,

S. Charnoz27, M. B. Davies28, M. Deleuil7, O. D. S. Demangeon8,19, B.-O. Demory5, D. Ehrenreich4, A. Erikson25,
L. Fossati20, M. Fridlund29,30, D. Futyan4, D. Gandolfi31, M. Gillon2, M. Guedel32, K. Heng6,33, K. G. Isaak17,
L. Kiss34,35,36, J. Laskar37, A. Lecavelier des Etangs38, M. Lendl4, C. Lovis4, D. Magrin39, P. F. L. Maxted40,
M. Mecina32,20, A. J. Mustill28, V. Nascimbeni39, G. Olofsson22, R. Ottensamer32, I. Pagano23, E. Pallé13,14,

G. Peter41, G. Piotto42,39, J.-Y. Plesseria43, D. Pollacco33, D. Queloz4,44, R. Ragazzoni42,39, N. Rando17,
H. Rauer25,45,46, I. Ribas15,16, N. C. Santos8,19, G. Scandariato23, D. Ségransan4, R. Silvotti47, A. E. Simon5,

A. M. S. Smith25, M. Steller20, G. M. Szabó48,49, N. Thomas5, S. Udry4, V. Viotto39, N. A. Walton50,
K. Westerdorff41, and T. G. Wilson26

(Affiliations can be found after the references)

Received 20 January 2021 / Accepted 14 April 2021

ABSTRACT

Context. Hot subdwarfs experienced strong mass loss on the red giant branch (RGB) and are now hot and small He-burning objects.
These stars constitute excellent opportunities for addressing the question of the evolution of exoplanetary systems directly after the
RGB phase of evolution.
Aims. In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes
(Kepler, K2, TESS, and CHEOPS) with our custom-made pipeline SHERLOCK in order to determine the occurrence rate of planets
around these stars as a function of orbital period and planetary radius. We also aim to determine whether planets that were previously
engulfed in the envelope of their red giant host star can survive, even partially, as a planetary remnant.
Methods. For this first paper, we performed injection-and-recovery tests of synthetic transits for a selection of representative Kepler,
K2, and TESS light curves to determine which transiting bodies in terms of object radius and orbital period we will be able to detect
with our tools. We also provide estimates for CHEOPS data, which we analyzed with the pycheops package.
Results. Transiting objects with a radius .1.0 R⊕ can be detected in most of the Kepler, K2, and CHEOPS targets for the shortest
orbital periods (1 d and shorter), reaching values as low as ∼0.3 R⊕ in the best cases. Sub-Earth-sized bodies are only reached for
the brightest TESS targets and for those that were observed in a significant number of sectors. We also give a series of representative
results for larger planets at greater distances, which strongly depend on the target magnitude and on the length and quality of the data.
Conclusions. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate
around hot subdwarfs. The Kepler, K2, and CHEOPS data will allow us to search for planetary remnants, that is, very close and small
(possibly disintegrating) objects.

Key words. planet-star interactions – planetary systems – stars: horizontal-branch – subdwarfs – techniques: photometric

1. Introduction

Hot subdwarf B (sdB) stars are hot and compact stars (Teff =
20 000–40 000 K and log g= 5.2–6.2; Saffer et al. 1994)
that lie on the blue tail of the horizontal branch (HB), that
is, the extreme horizontal branch (EHB). The HB stage corre-
sponds to core-He burning objects and follows the red giant

? CHEOPS data presented in Fig. 5 and lists presented in Appen-
dices C and D are only available at the CDS via anonymous ftp
to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.
u-strasbg.fr/viz-bin/cat/J/A+A/650/A205

branch (RGB) phase. Unlike most post-RGB stars that clus-
ter at the red end of the HB (the so-called red clump, RC)
because they lose almost no envelope on the RGB (Miglio
et al. 2012), sdB stars experienced strong mass loss on the
RGB and have extremely thin residual H envelopes (Menv <
0.01 M�, Heber 1986). This extremely thin envelope explains
their high effective temperatures and their inability to sus-
tain H-shell burning. This prevents these stars from ascending
the asymptotic giant branch (AGB) after core-He exhaustion
(Dorman et al. 1993). About 60% of the sdBs reside in binary
systems, and about half of them are in close binaries with
orbital periods of up to a few days (see, e.g., Allard et al. 1994;

Article published by EDP Sciences A205, page 1 of 19

https://www.aanda.org
https://doi.org/10.1051/0004-6361/202140381
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ftp://130.79.128.5
http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/650/A205
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A&A 650, A205 (2021)

Maxted et al. 2001), while the other half resides in
wider binaries with orbital periods of up to several years
(Stark & Wade 2003; Vos et al. 2018). Binary interactions
(through common-envelope, CE, evolution for the short orbits,
and stable Roche-lobe overflow, RLOF, evolution for the wide
orbits) are therefore the main reasons for this extreme mass
loss (Han et al. 2002, 2003). The hot O-type subdwarfs, or
sdO stars, have Teff = 40 000–80 000 K and a wide range of
surface gravities. The compact sdO stars (log g= 5.2−6.2) are
either post-EHB objects or direct post-RGB objects (through
a so-called late hot He-flash; Miller Bertolami et al. 2008), or
end products of merger events (Iben 1990; Saio & Jeffery 2000,
2002). The sdOs with log g < 5.2 are post-AGB stars, that is, stars
that have ascended the giant branch a second time after core-He
burning exhaustion (Reindl et al. 2016).

The formation of the approximately 40% of sdB stars that
appear to be single has been a mystery for decades. In the
absence of a companion, it is hard to explain how the star can
expel most of its envelope on the RGB and still achieve core-
He burning ignition. Recently, Pelisoli et al. (2020) suggested
that all sdB stars might originate from binary evolution. Merger
scenarios involving two low-mass white dwarfs have also been
investigated (Webbink 1984; Han et al. 2002, 2003; Zhang &
Jeffery 2012), but several facts challenge this hypothesis. First,
compact low-mass white dwarf binaries are quite rare, even
though some candidates are identified (Ratzloff et al. 2019). Sec-
ond, the mass distributions of single and binary sdB stars are
indistinguishable (Fontaine et al. 2012, Table 3 in particular).
This mass distribution is mainly obtained from asteroseismol-
ogy (some sdB stars exhibit oscillations, which allow the precise
and accurate determination of the stellar parameters, including
total mass; Van Grootel et al. 2013) and also from binary light-
curve modeling for hot subdwarfs in eclipsing binary systems.
Single and binary mass distributions peak at ∼0.47 M�, which
is the minimum core mass required to ignite He through a He-
flash at the tip of RGB (stars of &2.3 M� are able to ignite He
quietly at lower core masses, down to ∼0.33 M�, but the more
massive the stars, the rarer they are). A mass distribution of sin-
gle sdB stars from mergers, in contrast, would be much broader
(0.4−0.7 M�; Han et al. 2002). With the DR2 release of Gaia
(Gaia Collaboration 2018) and precise distances for many hot
subdwarfs (Geier 2020), it is now also possible to build a spec-
trophotometric mass distribution for a much larger sample than
what was achieved with the hot subdwarf pulsators or those
in eclipsing binaries. Individual masses are much less precise
than those obtained by asteroseismology or binary light-curve
modeling (Schneider et al. 2019). However, single and binary
spectrophotometric mass distributions share the same properties
here as well, which tends to disprove the hypothesis of different
origins for single and binary sdB stars. The third piece of evi-
dence against merger scenarios (which would most likely result
in fast-rotating objects) is the very slow rotation of almost all
single sdB stars, as obtained through v sin i measurements (Geier
& Heber 2012) or from asteroseismology (Charpinet et al. 2018).
Moreover, their rotation rates are in direct line with the core rota-
tion rates observed in RC stars (Mosser et al. 2012), which is
another strong indication that these stars and the single sdB stars
do share a same origin, that is, that they are post-RGB stars.

The question of the evolution of exoplanet systems after the
main sequence of their host is generally addressed by study-
ing exoplanets around subgiants, RGB stars, and normal HB
(RC) stars (hereafter the ’classical’ evolved stars). These classi-
cal evolved stars are typically very large stars, with radii ranging
from ∼5−10 R� to more than 1000 R�. This is much larger

than hot subdwarfs, which have radii in the range ∼0.1−0.3 R�
(Heber 2016). Their mass is typically higher than ∼1.5 M�, com-
pared to ∼0.47 M� for hot subdwarfs. The transit and radial
velocity (RV) methods are both challenging for these classical
evolved stars because the transit depth is diluted and there are
additional noise sources (Van Eylen et al. 2016). Another dif-
ficulty for the question of the fate of exoplanet systems after
the RGB phase itself is the difficulty of distinguishing RGB and
RC stars based on their spectroscopic parameters alone, which is
sometimes hard even with help of asteroseismology (Campante
et al. 2019). As a consequence, only large or massive plan-
ets are detected around the classical evolved stars (Jones et al.
2021, and references therein). A dearth of close-in giant planets
is observed around these evolved stars compared to solar-type
main-sequence stars (Sato et al. 2008; Döllinger et al. 2009).
This may be caused by planet engulfment by the host star,
but current technologies do not allow us to determine whether
smaller planets and remnants (such as the dense cores of for-
mer giant planets) are present. The lack of close-in giant planets
may also be explained by the intrinsically different planetary
formation for these intermediate-mass stars (see the discussion
in Jones et al. 2021). Ultimately, the very existence of planet
remnants may be linked to the ejection of most of the envelope
on the RGB that occurs for hot subdwarfs, while for classical
evolved stars, nothing stops the in-spiraling planet inside the host
star, and in all cases, the planet finally merges with the star,
is fully tidally disrupted, or is totally ablated by heating or by
the strong stellar wind. In other words, the ejection of the enve-
lope not only enables the detection of small objects as remnants,
but most importantly, it may even be the reason for the exis-
tence of these remnants by stopping the spiraling-in in the host
star.

Many studies have focused on white dwarfs (the ultimate
fate of ∼97% of all stars), including the direct observations of
transiting disintegrating planetesimals (Vanderburg et al. 2015),
the accretion of a giant planet (Gänsicke et al. 2019), and, most
recently, the transit of a giant planet (Vanderburg et al. 2020).
More than 25% of all single white dwarfs exhibit metal pol-
lution in their atmospheres (which should be pure H or He
because of the gravitational settling of heavier elements in these
objects with very high surface gravities), which is generally
interpreted as material accretion of surrounding planetary rem-
nants (Hollands et al. 2018, and references therein). Statistics
on the occurrence rate of planets around white dwarfs as a
function of orbital period and planet radius have also been estab-
lished (Fulton et al. 2014; van Sluijs & Van Eylen 2018; Wilson
et al. 2019). However, the vast majority of white dwarfs expe-
rienced two giant phases of evolution, namely the RGB and
the AGB. The AGB expansion and strong mass loss, followed
by the planetary-nebula phase, will have a profound effect on
the orbital stability of the surrounding bodies (e.g., Debes &
Sigurdsson 2002; Mustill et al. 2014; Maldonado et al. 2021).
Hence no direct conclusion concerning the effect of RGB expan-
sion alone on the exoplanet systems can be drawn from white
dwarfs.

Hot subdwarfs therefore constitute excellent opportunities
for addressing the question of the evolution of exoplanet systems
after the RGB phase of evolution. It is precisely this potential we
aim to exploit in this project by determining the occurrence rate
of planets around hot subdwarfs as a function of orbital period
and planet radius. We achieve this objective by performing a
transit search in all available light curves of hot subdwarfs from
space-based observatories, such as Kepler (Borucki et al. 2010),
K2 (Howell et al. 2014), TESS (Ricker et al. 2014), and CHEOPS

A205, page 2 of 19



V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

(Benz et al. 2021). In this first paper, we provide a review of the
current status of the search of planets around hot subdwarfs with
the different detection methods in Sect. 2. We present the obser-
vations and the tools we used to perform our transit search in
Sect. 3. We provide extensive tests of the photometric quality of
the light curves in Sects. 4 and 5, and we conclude and outline
future work in Sect. 6.

2. Search for planets around hot subdwarfs:
current status

To date, several planet detections around hot subdwarfs have
been claimed, but none of them received confirmation. With
the pulsation-timing method (variation in the oscillation peri-
ods of sdB pulsators), planets of a few Jupiter masses in orbits
at about 1 AU were announced around V391 Peg and DW
Lyn (Silvotti et al. 2007; Lutz et al. 2012), but these claims
have recently been refuted (Silvotti et al. 2018; Mackebrandt
et al. 2020). Based on weak signals that were interpreted as
reflection and thermal re-emission in Kepler light curves, five
very close-in (with orbital periods of a few hours) Earth-sized
planets have been claimed to orbit KIC 05807616 (Charpinet
et al. 2011) and KIC 10001893 (Silvotti et al. 2014). How-
ever, the attribution of these signals to exoplanets is debatable
(Krzesinski 2015; Blokesz et al. 2019). Using the RV method,
Geier et al. (2009) announced the discovery of a close-in
(Porb = 2.4 days) planet of several Jupiter masses around HD
149382, but this was ruled out by high-precision RV measure-
ments obtained with the Hobby-Eberly Telescope spectrograph,
which excluded the presence of almost any substellar compan-
ion with Porb < 28 days and M sin i & 1MJup (Norris et al. 2011).
No close massive planets (down to a few Jupiter masses) were
found from a mini RV survey carried out with the HARPS-N
spectrograph on eight apparently single hot subdwarfs (Silvotti
et al. 2020).

Several ground-based surveys with both photometric and RV
techniques target the red dwarf or brown dwarf close companions
to hot subdwarfs (Schaffenroth et al. 2018, 2019). These compan-
ions are frequent (Schaffenroth et al. 2018), but no Jupiter-like
planets have been found to date. In contrast, several discoveries
of circumbinary massive planets have been announced in close,
post-CE evolution sdB+dM eclipsing systems through eclipse-
timing variations, for instance, HW Vir, the prototype of the
class (Lee et al. 2009; Beuermann et al. 2012), NSVS 14256825
(Zhu et al. 2019), HS 0705+6700 (Pulley et al. 2015), NY Vir
(Qian et al. 2012), and 2M 1938+4603 (Baran et al. 2015). These
planets might correspond to first-generation, second-generation
(Schleicher & Dreizler 2014; Völschow et al. 2014), or hybrid
planets (which are formed from ejected stellar material that is
accreted onto remnants of first-generation planets; Zorotovic &
Schreiber 2013). All but one of the ten well-studied HW Vir sys-
tems show eclipse-timing variations (Heber 2016; Marsh 2018).
This may call for another explanation than planets (perhaps
something analogously to the mechanism suggested for eclipse-
timing variations in white dwarf binaries; Bours et al. 2016)
because the occurrence of circumbinary planets around close
main-sequence binaries that are the progenitors of such systems
is only ∼1% (Welsh et al. 2012). The properties of the claimed
planets often change or are discarded after new measurements
(Heber 2016; Marsh 2018), while the orbits are regularly found to
be dynamically unstable (e.g., Wittenmyer et al. 2013). None of
these claimed circumbinary planets has been confirmed through
another technique.

3. Observations and methods

3.1. Space-based light curves of hot subdwarfs

In the original Kepler field, 72 hot subdwarfs were observed at
the short cadence (SC) of 1 min for at least one quarter, including
the commissioning quarter Q0, which started on 2 May 2009.
During the one-year survey phase that followed Q0 (quarters Q1
of 33.5 days and Q2 to Q4 of 90 days each, divided into monthly
subquarters), 15 sdB stars were found to pulsate (Østensen et al.
2010, 2011). These 15 stars were consequently observed for the
rest of the mission at SC (with exceptions of some quarters for
some sdB pulsators, see the details in Table A.1). Three other
sdB pulsators, known as B3, B4, and B5, were found in the open
cluster NGC 6791 (Pablo et al. 2011) and were observed at SC
for various durations (see Table A.1). Of the non-pulsators, 47
B-type hot subdwarfs (sdB and sdOB) were observed for at least
one month at SC (5 of them for several quarters), as well as 7 sdO
stars. At the long cadence (LC) of 30 min, these 54 non-pulsating
hot subdwarfs were generally observed for several quarters, and
some of them for the whole duration of the mission. The list of
hot subdwarf targets in the original Kepler field and details on
the observing quarters in SC and LC can be found in Table A.1.
The primary Kepler mission stopped on 11 May 2013, during
Q17.2, after the failure of a second reaction wheel that was nec-
essary to stabilize the spacecraft and obtain the fine and stable
pointing for observations of the original field.

The Kepler mission was then redesigned as K2, for which the
two remaining reaction wheels allowed a stable pointing for ∼80
days of fields close to the ecliptic. An engineering test of 11 days
in February 2014 confirmed the feasibility of this strategy, and
19 campaigns (campaign 0–18) were executed from March 2014
to July 2018, when exhaustion of propellants definitively ended
the mission. When we account only for confirmed hot subd-
warfs, 39 sdB/sdOB pulsators were observed at SC through at
least one campaign in K2 fields. Two more sdB pulsators were
discovered through LC data only. Seventy-nine more sdB/sdOB
non-pulsators and 10 sdO non-pulsators were also observed at
SC. Finally, 44 hot subdwarfs were observed at LC only. In con-
trast to Kepler, the K2 SC and LC data generally cover only
one campaign (of about 80 days duration), although a few stars
were observed in two or three campaigns. The full list of hot
subdwarfs observed by K2 and details can be found in Table B.1.

TESS (Transiting Exoplanet Survey Satellite) has been oper-
ational since July 2018. It is performing a high-precision photo-
metric survey over almost the whole sky (about 90%), avoiding
only a narrow band around the ecliptic1. The TESS primary two-
year mission, which ended in early July 2020, consisted of 26
sectors that were observed nearly continuously for ∼27.4 days
each. Some overlap between sectors exists for the highest north-
ern and southern ecliptic latitudes, therefore some stars have
been observed for several sectors (see Table 1). The primary
mission TESS data products consist of SC observations sam-
pled every 2 min for selected stars, as well as full-frame images
(FFI) taken every 30 min that contain data for all stars in the
field of view. Accounting for confirmed hot subdwarfs alone,
1302 stars were observed for at least one sector at SC during
primary mission. This list was assembled by Working Group
(WG) 8 on compact pulsators of the TESS Asteroseismic Con-
sortium (TASC; see also Stassun et al. 2019). Table 1 presents
the statistics for these TESS primary mission observations of hot
subdwarfs, and the full list can be found online2 (see Appendix C

1 https://tess.mit.edu/observations/
2 https://github.com/franpoz/Hot-Subdwarfs-Catalogues

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A&A 650, A205 (2021)

Table 1. Statistics for the hot subdwarfs observed in the primary TESS
mission (July 2018–July 2020).

Number of sectors Number of stars

1 877
2 205
3 72
4 23
5 21
6 24
7 7
8 10
9 6
10 6
11 13
12 23
13 15

G magnitude Number of stars

8–9 3
9–10 4
10–11 18
11–12 60
12–13 162
13–14 278
14–15 384
15–16 341
16–17 51
Beyond 17 1

for details). The TESS extended mission started on 4 July 2020,
and revisits all sectors for the same duration. The sectors are
referred to with increasing numbers (Sector 27, 28, etc.). An
‘ultra short cadence’ of 20 s is now available in addition to the
normal SC of 2 min, and FFIs are now taken every 10 min. After
release of Sectors 27–31, 243 confirmed hot subdwarfs have been
observed at 20 s cadence, and 670 more at a 2-min cadence (these
targets were also selected by WG8 of the TASC). Most of these
targets were previously observed in the primary mission (sectors
1–26), but about one-third are new targets that were not observed
during the primary mission3 (see Appendix D for details). It is
expected that about 2300 hot subdwarfs will have been observed
at the end of the two-year extended mission.

CHEOPS (CHaracterising ExOPlanets Satellite) is a Euro-
pean Space Agency (ESA) mission primarily dedicated to the
study of known extrasolar planets orbiting bright (6<V< 12)
stars. It was successfully launched into a 700 km altitude Sun-
synchronous 99-min orbit on 18 December 2019. CHEOPS is
a 30 cm (effective) aperture telescope optimized for obtaining
high-cadence high-precision photometric observations for one
single star at a time in a broad optical band. CHEOPS is a
pointed mission with mostly time-critical observations, there-
fore it has ∼20% of free orbits that are partly used to observe
bright apparently single hot subdwarfs as a “filler” program (pro-
gram ID002). This means that hot subdwarf observations are
carried out when CHEOPS has no time-constrained or higher-
priority observations. The selected targets are generally close
to the ecliptic where CHEOPS has its maximum visibility (but
they were not observed by K2) and are observed for one to three

3 The list can be found at https://github.com/franpoz/
Hot-Subdwarfs-Catalogues

consecutive orbits with the goal of a total of 18 orbits per target
per season. As of 19 December 2020 (eight months after start-
ing the program), 46 hot subdwarf targets have been observed
by CHEOPS for a total of 290 orbits. The exposure time is 60
sec for all these targets (except for HD 149382, for which it is
41 s). The list of CHEOPS targets and details can be found in
Table 2 (the columns “Phase coverage” and “Minimum planet
size” are explained in Sect. 5), and online4. The nominal dura-
tion of the CHEOPS mission is 3.5 yr (i.e., until the end of
2023), when we hope to reach a total of about 25–30 orbits on
50–60 targets.

Finally, for completeness, we mention that the CoRoT satel-
lite (Baglin et al. 2006) performed for ∼24 days a high-quality,
nearly continuous photometric observation of the sdB pulsator
KPD 0629-0016 (Charpinet et al. 2010). We will add these
observations in our transit survey, but we did not carry out a
performance test for this one star here.

Figure 1 (top) summarizes the available sample of hot subd-
warf space-based light curves ranked per bin of G magnitudes,
as obtained from Kepler, K2, TESS (primary mission, as well
as hot subdwarfs observed for the first time in the extended mis-
sion), and CHEOPS (as of 19 December 2020). No G magnitude
is available for a few Kepler and K2 targets, therefore they are
included in Fig. 1 with Kp minus 0.1, which is the mean dif-
ference between Kp and G magnitudes observed for targets for
which both estimates are available. Figure 1 (bottom) shows the
celestial distribution of this sample.

3.2. Tools for transit searches in space-based light curves

To search for transit events, we will make use of our custom
pipeline SHERLOCK (Pozuelos et al. 2020)5. This pipeline pro-
vides the user with easy access to Kepler, K2, and TESS data
for both SC and LC. The pipeline searches for and downloads
the pre-search data conditioning simple aperture (PDC-SAP)
flux data from the NASA Mikulski Archive for Space Telescope
(MAST). Then, it uses a multi-detrend approach in the WOTAN
package (Hippke et al. 2019), whereby the nominal PDC-SAP
flux light curve is detrended several times using a biweight filter
or a Gaussian process, by varying the window size or the ker-
nel size, respectively. This multi-detrend approach is motivated
by the associated risk of removing transit signals, in particu-
lar, short and shallow signals. Each of the new detrended light
curves, jointly with the nominal PDC-SAP flux, is then pro-
cessed through the TRANSIT LEAST SQUARES (TLS) package
(Hippke & Heller 2019) in the search for transits. In contrast to
the classical box least-squares (BLS) algorithm (Kovács et al.
2002), the TLS algorithm uses an analytical transit model that
takes the stellar parameters into account. Then, it phase folds the
light curves over a range of trial periods (P), transit epochs (T0),
and transit durations (d). It then computes the χ2 between the
model and the observed values, searching for the minimum χ2

value in the 3D parameter space (P, T0 , and d). The TLS algo-
rithm has been found to be more reliable than the classical BLS
in finding any type of transiting planet, and it is particularly well
suited for the detection of small planets in long time series, such
as those coming from Kepler, K2, and TESS. The TLS algo-
rithm also allows the user to easily fine-tune the parameters to
optimize the search in each case, which is particularly interesting

4 https://github.com/franpoz/Hot-Subdwarfs-Catalogues
5 The SHERLOCK code (Searching for Hints of Exoplanets from
Lightcurves of space-based seekers) is fully available at the GitHub site
https://github.com/franpoz/SHERLOCK

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https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/SHERLOCK


V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

Table 2. Hot subdwarf targets observed by CHEOPS (as of 19 December 2020).

Name Type G mag # of orbits Phase coverage Min. planet size
(as of 19 Nov. 2020) (days, for >80% coverage) (R⊕, for S/N = 5 and 0.18 R� host)

Active
HD 149382 sdB 8.80 14 (7× 2) 0.47 0.4
HD 127493 sdO 9.96 6.8 (2× 1 + 4.8) 0.18 0.4
TYC 981-1097-1 sd 12.01 18 (6× 3) 0.68 0.7
Feige 110 sdOB 11.79 6 (3× 2) 0.25 0.7
CW83-1419-09 sdOB 12.04 12 (4× 3) 0.39 0.7
EC 14248-2647 sdOB 11.98 2 (1× 2) <0.10 0.7
PG 2219+094 sdB 11.90 5 (5× 1) 0.18 0.7
PG 1352-023 sdOB 12.06 6 (3× 2) 0.18 0.8
LS IV -12 1 sdO 11.11 4 (4× 1) 0.18 0.8
Feige 14 sdB 12.77 5 (5× 1) 0.11 0.8
EC 22081-1916 sdB 12.94 6 (3× 2) 0.25 0.8
LS IV+06 2 He-sdO 12.14 5 (5× 1) 0.18 0.8
MCT 2350-3026 sdO 12.07 8 (4× 2) 0.32 0.8
TYC 982-614-1 sd 12.21 18 (6× 3) 0.68 0.8
EC 20305-1417 sdB 12.34 6 (2× 3) 0.25 0.8
LS IV+109 He-sdO 11.97 10 (5× 2) 0.39 0.8
PG 1432+004 sdB 12.75 4 (2× 2) 0.11 0.8
TonS403 sdO 12.92 11 (11× 1) 0.25 0.8
TYC 497-63-1 sdB 12.89 5 (5× 1) 0.11 0.8
TYC 999-2458-1 sdB 12.59 3 (1× 3) 0.18 0.9
TYC 499-2297-1 sdB 12.63 12 (6× 2) 0.54 0.9
LS IV+00 21 sdOB 12.41 4 (2× 2) 0.18 0.9
PG 1245-042 sd 13.60 7 (7× 1) 0.18 1.0
PG 2151+100 sdB 12.68 9 (3× 3) 0.39 1.0
EC 13047-3049 sdB 12.78 2 (1× 2) <0.10 1.0
PG 1505+074 sdB 12.37 2 (2× 1) <0.10 1.0
LS IV -14 116 He-sdOB 12.98 2 (2× 1) 0.11 1.0
EC 12578-2107 sdB 13.52 7 (7× 1) 0.25 1.0
EC 13080-1508 sdB 13.65 3 (3× 1) 0.18 1.0
PB 8783 sdO+F 12.23 6 (3× 2) 0.25 1.1
MCT 2341-3443 sdB 10.92 4 (2× 2) 0.18 1.1
EC 21595-1747 sdOB 12.62 4 (2× 2) 0.18 1.1
PG 1230+067 He-sdOB 13.12 2 (1× 2) 0.11 1.1
EC 15103-1557 sdB 12.82 6 (3× 2) 0.25 1.1
PG 2313-021 sdB 13.00 6 (3× 2) 0.18 1.1
PG 2349+002 sdB 13.27 10 (1× 10) 0.32 1.1
PG 1207-033 sdB 13.34 2 (2× 1) 0.11 1.1
PG 1303-114 sdB 13.63 5 (5× 1) 0.18 1.1
PG 1343-102 sdB 13.69 4 (4× 1) 0.25 1.1
Suspended
LS IV+06 5 sdB 12.37 8 (4× 2) – >1.3
EC 14338-1445 sdB 13.55 2 (2× 1) – >1.5
EC 14599-2047 sdB 13.57 3 (3× 1) – >1.5
EC 01541-1409 sdB 12.27 12 (4× 3) – >1.5
TYC 1077-218-1 sdOB 12.41 3 (3× 1) – >2.0
LS IV +09 2 sdB 12.69 4 (2× 2) – >2.0
TYC 467-3836-1 sdB 11.70 6 (6× 1) – >2.0

for shallow transits. In addition, SHERLOCK incorporates a vet-
ting module that combines the TPFplotter (Aller et al. 2020),
LATTE (Eisner et al. 2020), and TRICERATOPS (Giacalone et al.
2021) packages, which allows the user to explore any contamina-
tion source in the photometric aperture used, momentum dumps,
background flux variations, x–y centroid positions, aperture size
dependences, flux in-and-out transits, each individual pixel of
the target pixel file, and to estimate the probabilities for differ-
ent astrophysical scenarios such as transiting planet, eclipsing

binary, and eclipsing binary with twice the orbital period. Col-
lectively, these analyses help the user estimate the reliability of a
given detection.

For each event that passes the vetting process, the user may
wish to perform ground- or space-based follow-up observations
to confirm the transit event on the target star. This is particu-
larly critical for TESS observations because of the large pixel
size (21 arcsec) and point spread function (which can be as
large as 1 arcmin). These aspects increase the probability of

A205, page 5 of 19



A&A 650, A205 (2021)

Fig. 1. Top: number of hot subd-
warfs per G-magnitude bin observed
by Kepler, K2, TESS (hot subdwarfs
observed in primary mission, which are
almost all reobserved in the extended
mission), TESS extended only (hot sub-
dwarfs observed for the first time in
the extended mission; S27 to S32), and
CHEOPS (as of 19 December 2020).
Bottom: celestial distribution of these
hot subdwarfs: TESS primary mission
(blue dots), TESS extended mission 2
min and 20 s (red and dark orange dots),
Kepler (purple crosses), K2 (black tri-
angles), and CHEOPS (green stars).

contamination by a nearby eclipsing binary (see, e.g., Günther
et al. 2019; Kostov et al. 2019; Quinn et al. 2019; Nowak et al.
2020). However, the results coming directly from the searches
performed with SHERLOCK through the TLS algorithm are not
optimal; that is, the associated uncertainties of P, T0, and d
are large, and their temporal propagation makes using them to
compute future observational windows and to schedule a follow-
up campaign impractical. SHERLOCK therefore uses the results
coming from TLS as priors to perform model fitting, inject-
ing them into allesfitter (Günther & Daylan 2019, 2021).
The user can then choose between nested sampling or a Markov
chain Monte Carlo (MCMC) analysis, whose posterior distribu-
tions are much more refined, with significant reductions of a
few orders of magnitude of the uncertainties of P, T0, and d.
This allows us to schedule a follow-up campaign for which the
observational windows are more reliable.

4. Injection-and-recovery tests

To quantify the detectability of transiting bodies in our sample of
hot subdwarfs, we performed a suite of injection-and-recovery
tests. While the detectability of a transit depends on the target
and on the sector or quarter, these experiments allowed us to
verify the general reliability of our survey. We explored several
data sets coming from the Kepler, K2, and TESS missions. For
each one, we chose a range of stellar magnitudes that we studied.

In all cases, we followed the procedure described by Pozuelos
et al. (2020) and Demory et al. (2020); that is, we downloaded
the PDC-SAP fluxes in each case and generated a grid of syn-
thetic transiting planets by varying their orbital periods and radii,
which were injected in the downloaded light curves. We then
detrended the light curves and searched for the injected plan-
ets. The search itself was done by applying the simple TLS
algorithm. The multi-detrend approach applied by SHERLOCK
makes it more efficient at finding shallow-periodic transits, but
with a higher computational cost. The full use of SHERLOCK in
the injection-and-recovery experiments is therefore too expen-
sive. This means that our findings in these experiments might be
considered as upper limits for the minimum planet sizes, and
during our survey, we might detect even smaller planets. We
defined a synthetic planet as “recovered” when we detect its
epoch with a one-hour accuracy and if we find its period with
an accuracy better than 5%. Depending on the number of avail-
able sectors or quarters, we explored the Rplanet–Pplanet parameter
space in different ranges. We conducted two different experi-
ments to qualify the performances that can be achieved with
Kepler, K2, and TESS data. The first experiment consisted of full
injection-and-recovery tests focusing on a region of the parame-
ter space corresponding to small close-in exoplanets. For Kepler
and K2, the injected planet range was 0.3–1.0 R⊕ with steps of
0.1 R⊕, and 0.5–4.1 d with steps of 0.2 d for a total of 152 scenar-
ios. For TESS, the injected planets have 0.5–3.0 R⊕ with steps of

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V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

Fig. 2. Injection-and-recovery tests. Left panel: KIC 8054179 (Kp = 14.40, G = 14.34), based on Q6 Kepler data (90 days). Right panel: EPIC
206535752 (Kp = 13.99, G = 14.10), observed during Campaign 3 of K2 (81 days). Injected transits of planets have 0.3–1.0 R⊕ (steps of 0.1 R⊕) with
0.5–4.1 d (steps of 0.2 d) orbital periods.

0.05 R⊕, and 1.0–6.0 d with steps of 0.1 d, that is, a total of 2500
scenarios for each of them. However the six-sector test was made
with 0.5–2.5 R⊕ with steps of 0.2 R⊕, and 1.0–5.0 d with steps
of 0.2 d, for a total of 200 scenarios for computer-cost reasons
(as a corollary, injection-and-recovery tests on more sectors, up
to 13 sectors for one-year-continuous observations, are beyond
our reach). The second experiment concerned larger planets at
greater distances, that is, up to 10.0 R⊕ and 35 d orbital period.
Full injection-and-recovery tests with sufficiently small steps led
to a too high computational cost. We instead chose to focus on
particular periods (1, 5, 15, 25, and 35 d) and determined the
minimum planet size detectable for each set of data considered
for these periods. This limit of 35 d is justified by the tran-
sit probability, which quickly decreases to very low values with
increasing orbital period (for a typical hot subdwarf of 0.15 R�
and 0.5 M�, the transit probabilities at 10 d and 50 d are about
1 and 0.35%, respectively). For each period, we explored ∼30
scenarios with fine steps of 0.1 d and 0.1 R⊕, around a nominal
value of the size that was previously computed with an explo-
ration of sizes from 1 to 10 R⊕ with steps of 1 R⊕. This strategy
allowed us to obtain robust estimates of the sizes with a recovery
rate &90% for each explored period. The total number of scenar-
ios is considerably higher than for the full injection-and-recovery
maps, as those shown in Figs. 2−4. The results from these exper-
iments are consequently better founded than those from the full
maps, which might be considered as more rough estimates of the
recovery rates, but with a better and quicker general overview
for small and close-in exoplanets. We do not exclude that longer-
period planets might be found during our transit survey. For the
tests carried out here, however, whose purpose is to quantify our
potential of finding planets with a transit survey, the limit of 35 d
is a good balance between the computational cost and the proba-
bility of transit. For Kepler data we explored one quarter, which
corresponds to ∼90 d of data, as well as monthly subquarters. A
similar approach was adopted for K2 data, where observations
usually span one campaign of ∼ 80 d of data and subsamples of
30 d. Finally, for TESS data, we tested data covering one, two,
three, and six sectors (27–162 d).

In all experiments, we assumed that the host star is a canoni-
cal hot subdwarf with a radius of 0.175± 0.025 R� and a mass of

0.47± 0.03 M�. We considered only SC data here. We selected
targets that are as unremarkable as possible, that is, nonvariable
stars (i.e., no peak emerges above 4σ in a Lomb-Scargle peri-
odogram, from pulsations, from reflection or ellipsoidal effect
due to a binary nature, or from any other type of variability) with
quiet (low scatter) light curves. The experiments performed here,
based on the injection of synthetic transits, for computational
cost reasons applied only one detrending to the resulting light
curves. In all cases we used the biweight method with a nomi-
nal window-size of 2.5 h, which is large enough to cover short
transits of close-in exoplanets (which have a typical duration of
∼20 min) and to remove most of the stellar noise, variability,
and instrumental drifts. For the actual transit search, the light
curves will be detrended 12 times using either a biweight filter
or a Gaussian process (Sect. 3.2), which allows us to optimize
the planet search and increase the detectability of small planets.
In this context, it is therefore important for our injection-and-
recovery experiments here to select targets that are as quiet as
possible (minimizing the need of detrending) in order to obtain
results that are as representative as possible.

4.1. Results for Kepler and K2

Figure 2 (left) shows the full injection-and-recovery test for
KIC 8054179 (Kp = 14.40, G = 14.34) from the Q6 Kepler data
(90 days). We found that planets smaller than ∼0.4 R⊕ with
orbital periods longer than ∼1.5 days and smaller than ∼0.5 R⊕
with orbital periods longer than ∼3.2 days have recovery rates
below 50%, that is, we will most likely be unable to detect them
(Fig. 2). For the shortest orbital periods (.1.5 d), objects as small
as ∼0.3 R⊕ are fully recovered6. Results from the second experi-
ment focusing on larger planets at greater distances are presented
in Table 3: the smallest planet that can be detected for 1–35 d
increases from 0.3 to 1.2 R⊕ for KIC 8054179, considering 90 d
of data.

We also performed similar experiments for four other rep-
resentative Kepler targets with increasing magnitudes for one
subquarter, that is, one month of data (we also provide results
6 We explicitly checked that the detection rate of planets below ∼0.3 R⊕
quickly falls below 50%.

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Table 3. Minimum size of planets in units of R⊕ that can be detected in
typical light curves with a &90% recovery rate.

Object ID G Mag Data 1 d 5 d 15 d 25 d 35 dlength (d)

Kepler
8054179 14.3 90 0.3 0.5 0.8 1.0 1.2

30 0.5 0.6 1.0 – –
3353239 15.2 30 0.6 0.8 1.1 – –
5938349 16.1 30 0.7 1.1 2.0 – –
8889318 17.2 30 0.9 1.2 2.4 – –
5342213 17.7 30 1.2 1.7 3.2 – –

K2
206535752 14.1 80 0.6 0.8 1.0 1.5 2.1

30 0.6 0.9 1.6 – –
211421561 14.9 30 0.7 1.4 1.9 – –
228682488 16.0 30 1.0 1.4 2.5 – –
251457058 17.1 30 1.4 2.3 3.4 – –
248840987 18.1 30 2.1 3.3 5.4 – –

TESS
147283842 10.1 27 0.5 0.7 1.5 – –
362103375 13.0 27 1.0 1.7 2.0 – –

162 0.7 0.8 0.9 1.0 1.3
096949372 13.0 27 1.1 1.8 2.0 – –
441713413 13.1 27 1.3 1.7 2.0 – –

54 1.3 1.7 1.9 >10 >10
085400193 14.1 27 1.8 2.3 2.8 – –
220513363 14.1 27 1.6 1.8 2.7 – –

81 1.3 1.6 2.5 3.0 3.0
000008842 15.0 27 2.7 3.2 4.7 – –

Notes. All stars have 0.175± 0.025 R� and 0.47± 0.03 M�.

for one month data for KIC 8054179, for comparison purposes).
The results are presented in Table 3. For a typical Kepler tar-
get of 16th G magnitude (see Fig. 1), a sub-Earth-size planet of
0.7 R⊕ can still be detected at a 1 d period and a 2.0 R⊕ at 15 d
period (considering one month of data).

Figure 2 (right) shows the full injection-and-recovery test for
EPIC 206535752 (Kp = 13.99, G = 14.10), which was observed
during Campaign 3 of K2 (81 days). We find that ∼0.6 R⊕ plan-
ets are fully recovered up to ∼3 d orbital periods, while the
detectability of objects smaller than 0.5 R⊕ quickly drops below
50% for all orbital periods, meaning that we will likely not be
able to detect them. Results from the second experiment on
EPIC 206535752 focusing on larger planets at greater distances
are presented in Table 3: the smallest planet that can be detected
for 1 d to 35 d quickly increases from 0.6 to 2.1 R⊕, considering
80 d of data.

Similar experiments were also carried out for four other K2
targets with increasing magnitudes with a subsample of 30 d
data. The results are presented in Table 3. For a typical K2 tar-
get of 15th G magnitude (see Fig. 1), a sub-Earth-size planet of
0.7 R⊕ can still be detected at a 1 d period, as can a 1.9 R⊕ at 15 d
period (considering one month of data).

As a concluding remark, for a given magnitude and data
duration, the Kepler performances are significantly superior to
those of K2, although the K2 targets are generally brighter (Fig. 1
and Table 3). Almost all Kepler targets will allow us to detect
transiting objects with a radius .1.0 R⊕. This is the case for about
two-thirds of the K2 targets.

4.2. Results for TESS

Figure 3 presents results of injection-and-recovery tests for four
stars observed in one sector by TESS. The four selected stars
also are very quiet, non-variable stars. They have magnitudes
of G = 10.1, G = 13.0, G = 14.1, and G = 15.0. Figure 3 shows
that typically, ∼0.5 R⊕ (G∼ 10.0), ∼1.2 R⊕ (G∼ 13.0), ∼1.9 R⊕
(G∼ 14.1), and ∼2.7 R⊕ (G∼ 15.0) planets can be retrieved from
TESS one-sector light curves for the shortest orbital periods with
a &90% recovery rate.

To appreciate the increase in detectability with multisec-
tor observations, we performed similar tests on TIC 441713413
(G = 13.07), which was observed in two sectors (S16 and S23),
on TIC 220513363 (G = 14.1), which was observed in three sec-
tors (S1, S2, and S3), and on TIC 362103375 (G = 13.04), which
was observed in six sectors (S14, S15, S18, S22, S25, and S26).
All stars were compared to results from one-sector-only tests
(S16 for TIC 441713413, S1 for TIC 220513363, and S14 for
TIC 362103375). Figure 4 and Table 3 present and compare the
results of these experiments. The improvement in detectability
from one to two sectors is barely perceptible and is noticeable
only for orbital periods beyond 5 d. This is an important result
because the majority of TESS targets were observed for one sec-
tor only during the primary mission (Table 1), and it will be
reobserved for another one sector in the extended mission. No
significant improvement in detectability obtained from the one-
sector primary mission (Fig. 3) is therefore expected with one
more sector data in the extended mission. The improvement from
one to three sectors (TIC 220513363, see Table 3) and six sec-
tors (TIC 362103375, see Fig. 4 and Table 3) is increasingly
noticeable: We are now able to reach sub-Earth-sized objects
up to 25 d with 6 sectors, for example, which was only pos-
sible for an orbital period of 1 d (and below) with one sector
only.

The effect of the data length on the minimum detectable
radius can be described further. In an ideal case, the longer the
data set, the smaller the planet that can be detected because
of the increased number of stacked transits. This improves the
statistics and increases the signal-to-noise ratio (S/N). This is
directly related to the working procedure of our transit-search
algorithm (TLS, see Sect. 3.2). However, the real nature of the
light curves, which always present a level of noise that cannot
be removed, means that we do not always have a clear improve-
ment when more transits are stacked. This is in particular the
case for the short orbital periods. Adding more transits does not
always yield a vast improvement, providing there is already a
large number of them. This is shown in Table 3 for orbital peri-
ods of 1 d, for instance, for KIC 8054179, EPIC 206535752, and
TIC 441713413. For longer orbital periods, the effect is stronger
because the increase in the number of stacked transits is rela-
tively more important. For example, for the TESS sample, the
improvement in the minimum size of planets that can be detected
with an orbital period of 15 d is remarkable when we expand
our analysis from one sector to two (TIC 441713413), three (TIC
220513363), and six sectors (TIC 362103375).

To conclude this section, we mention that Figs. 2−4 as well
as Table 3 also allow us to assess the general reliability of our
results for the Kepler, K2, and TESS light curves. While the
detectability will (unavoidably) be dependent on the target (for
similar magnitude) and/or sector or quarter (for similar data
length; also because of the actual radius of the host star), the
general comparison of the tests carried out here shows con-
sistent trends. For example, the results for three different stars
with G∼ 13.0 (TIC 096949372, 362103375, and 441713413) for

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V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

Fig. 3. Results of injection-and-recovery tests for four sdB stars observed in one sector by TESS: TIC 147283842 (G = 10.1, top left panel),
TIC 96949372 (G = 13.0, top right panel), TIC 85400193 (G = 14.1, bottom left panel), and TIC 000008842 (G = 15.0, bottom right panel). 2500
injection-and-recovery tests were made for each star.

one-sector TESS data of three different sectors are globally
consistent.

5. CHEOPS performances for hot subdwarfs

Figure 5 displays typical light curves obtained by CHEOPS for
four representative targets: (1) HD 149382, one of the brightest
known sdB stars (G = 8.9), which was not observed by TESS,
Kepler, or K2; (2) CW83-1419-09 (G = 12.0), and (3) TYC 982-
614-1 (G = 12.2), which represent typical CHEOPS targets in
terms of magnitude; and (4) TYC 499-2297-1, a fainter target of
G = 12.6, which exceeds the CHEOPS standard specifications.
The light curves were processed using version 12 of the Data
Reduction Pipeline (DRP; Hoyer et al. 2020).

These light curves were obtained with the aperture that offers
the smallest root mean square of variation in count rates, which
is generally the DEFAULT one (which has a radius of 25 arcsec).
Then, we evaluated how the flux was correlated with different
parameters such as the time, the CHEOPS roll angle, the x–y
centroids, the background, and the contamination. This inspec-
tion was made with the pycheops7 package (v0.9.6), which
is developed specifically for the analysis of CHEOPS data.
We thus decorrelated the light curves of any undesired trends

7 https://github.com/pmaxted/pycheops

by calculating the Bayesian information criteria (BIC) of each
combination of trends under the assumption that the combina-
tion that induces the lowest BIC describes any trends best. We
also removed outliers as required. After the light curves were
decorrelated, we visually inspected them in the search for poten-
tial transits.

The hot subdwarf observations made by CHEOPS are fillers.
This results in light curves spanning 1.5–5 h (with gaps due to
Earth occultations and/or passages through the South Atlantic
Anomaly, but always with an efficiency of at least 60% for an
orbit, and always less than 100%) separated by several days
for a given target. This makes the application of injection-and-
recovery tests as conducted in Sect. 4 impractical. Our injection-
and-recovery experiments were conducted with the TLS transit
search tool, which is useful for long time-series observations
such as those coming from Kepler, K2, or TESS. The power
of TLS-based searching relies on the stacking of many transits,
which eventually increases the S/N of a given periodic signal.
However, the CHEOPS light curves are short observational data
sets, in which we expect to find single transits. To characterize
the CHEOPS performance for hot subdwarfs, we therefore esti-
mated the minimum planet size based on the transit depth that
could be detected with an S/N of 5 assuming a transit duration of
20 min (which is the typical duration for a ∼12 h orbital period)
and for various typical stellar radii from 0.15 to 0.20 R�.

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A&A 650, A205 (2021)

Fig. 4. Results of injection-and-recovery tests for stars observed in multiple sectors by TESS. Top panels: TIC 441713413 (G = 13.07), one-sector
data (left) and two-sector data (right). Bottom panels: TIC 362103375 (G = 13.04), one-sector data (left) and six-sector data (right).

The noise of the light curve is estimated with the pycheops
package using the scaled noise method. It assumes that the
noise in the light curve is white noise with a standard error b
times the error values provided by the CHEOPS DRP. We then
inject transits into the light curve and find the transit depth such
that the S/N of the transit depth measurement is 1. The tran-
sit model used for this noise estimate includes limb darkening,
therefore we define the depth as D = k2, where k is the planet-
star radius ratio used to calculate the nominal model. We can
use a factor s to modify the transit depth in a nominal model
m0 calculated with approximately the correct depth to produce a
new model m(s) = 1 + s× (m0 − 1). If the data are normalized
fluxes f = f1, . . . , fN with nominal errors σ=σ1, . . . , σN , then
the log-likelihood for the model given the data is

lnL=−
1

2b2 χ
2 −

1
2

N∑
i = 1

lnσ2
i − N ln b −

N
2

ln(2π),

where χ2 =
∑N

i
(
fi − 1 − s(m0,i − 1

)2 /σ2
i . The maximum likeli-

hood occurs for parameter values ŝ and b̂ such that ∂ lnL
∂s

∣∣∣
ŝ,b̂ = 0

and ∂ lnL
∂b

∣∣∣
ŝ,b̂ = 0, from which we obtain

ŝ =

N∑
i = 1

( fi − 1)(m0,i − 1)
σ2

i

 N∑
i = 1

(m0,i − 1)2

σ2
i

−1

and

b̂ =

√
χ2/N.

The standard errors on the eclipse depth if s ≈ 1 are

σD = Db

 N∑
i = 1

(mi − 1)2

σ2
i

−1/2

.

Figure 6 shows the minimum planet sizes that can be detected
at S/N = 5 for our four representative targets. For HD 149382, a
∼0.4 R⊕ object (for a 0.18 R� host) could be detected at S/N = 5
if it is transiting. CW83-1419-09 and TYC 982-614-1 exhibit
typical results for CHEOPS targets, reaching detections of ∼0.7–
0.8 R⊕ objects. Finally, the fainter TYC 499-2297-1 could allow
the detection of a ∼0.9 R⊕ object. The minimum planet sizes for
all CHEOPS targets can be found in Table 2. They are given for
a 0.18 R� host and for S/N = 5 in all cases.

Another important property to determine because of the filler
nature of CHEOPS observations is which orbital periods (and for
which coverage of the orbit) are reached with the existing obser-
vations. This was measured by computing the phase coverage of
a hypothetical planet in a range of periods. More precisely, we
computed the percentage of the phase covered for each orbital
period from Porb = 0.001 d to 5 d in intervals of 0.001 d. To do

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V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

0.04 0.06 0.08 0.10 0.12 0.14 0.16
BJD-2459020

0.999

1.000

1.001

No
rm

al
ise

d 
flu

x

HD 149382

Decorrelation model
DRP data

0.04 0.06 0.08 0.10 0.12 0.14 0.16
BJD-2459020

Decorrelated data
5-min bin

0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400
BJD-2458962

0.996

0.998

1.000

1.002

1.004

No
rm

al
ise

d 
flu

x

CW83-1419-09

0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400
BJD-2458962

0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850
BJD-2459027

0.9950

0.9975

1.0000

1.0025

1.0050

No
rm

al
ise

d 
flu

x

TYC 982-641-1

0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850
BJD-2459027

0.26 0.28 0.30 0.32 0.34 0.36 0.38
BJD-2459064

0.990

0.995

1.000

1.005

1.010

No
rm

al
ise

d 
flu

x

TYC 499-2297-1

0.26 0.28 0.30 0.32 0.34 0.36 0.38
BJD-2459064

Fig. 5. Representative light curves of hot subdwarfs produced by CHEOPS. From top to bottom: HD 149382 (G = 8.9) in its fifth visit, CW83
1419-09 (G = 12.0) in its first visit, TYC 982-6141 (G = 12.2) in its fourth visit, and TYC 499-2297-1 (G = 12.6) in its fourth visit. In all cases, the
raw light curves as processed by the DRP (gray dots) are displayed in the left panels, jointly with the best decorrelation model (orange line) found
by means of the pycheops package. In the right panels, the decorrelated data (gray dots) with a 5-min bin (blue dots) are shown. The y-scale is
the same for each pair of right and left panels.

this, we evaluated the phase coverage for a total of 5000 periods.
Then, to aid interpreting the phase coverage at different peri-
ods, we binned the periods by 1.7 h. To illustrate the current
status of our observational program, we estimated the period
at which a phase coverage of ∼80% is reached for each target,
meaning that periods equal to or shorter than this would most
likely be detected if the planet exists and transits. However, even
when the probabilities are low, a hypothetical planet may still
reside in the unexplored phase. Results for our four representa-
tive targets are presented in Fig. 7. As of 19 December 2020, we
found a phase coverage of ∼80% for orbital periods of ∼0.47 d,
∼0.39 d, ∼0.68 d, and ∼0.54 d for our four representative cases
HD 149384 (7 times two orbits), CW83 1419-09 (4 times three
orbits), TYC 982-614-1 (6 times three orbits), and TYC 499-
2297-1 (6 times two orbits). The orbital periods reached for a
phase coverage higher than 80% for the CHEOPS targets can be
found in Table 2.

In light of the results of the injection-and-recovery tests in
the Kepler, K2, and TESS light curves, all CHEOPS targets with
a minimum detectable planet size greater than &1.1R⊕ have been
suspended (see Table 2). These targets generally have fainter
magnitudes, are located in a crowded field, or have a bright
close contaminating object, which explains the poorer ability of
detecting planets around these objects. Another explanation is
that some targets are pressure-mode (p-mode) sdB pulsators with
a relatively high amplitude (this is the case for EC 15041-1409
and TYC 1077-218-1), which are not properly removed with our

current detrending procedure (this is an improvement we aim to
implement in the coming months). We instead chose to focus on
the most promising targets for which planets below .1.1 R⊕ can
be detected because in these cases, CHEOPS will notably con-
tribute to increasing the number of targets for which we could
detect planetary remnants (which are likely small, possibly dis-
integrating objects) around post-RGB stars. From Tables 1, A.1,
and B.1 and the results from Table 3, it is estimated that about
160 stars observed by Kepler and K2 (almost all of them for
Kepler, and about two-thirds of them for K2) and about 50 stars
from TESS (the very brightest ones, and those with G . 13.0
observed for at least about six sectors), will reach this minimum
planet size. Statistically, only ∼40% of them are single hot sub-
dwarfs, while in contrast, all CHEOPS targets have been chosen
to be single hot subdwarfs (or, in a few cases, subdwarfs in wide
binary systems) to the best of our knowledge.

The orbital periods reached by the CHEOPS filler observa-
tions will remain modest (about 1 d orbital period with a 80%
phase coverage by the end of the mission for most targets). How-
ever, these results are valuable for placing constraints on the
survival rates of planets that are engulfed in the envelope of their
red giant host. These remnants, if present, are expected to have
very short orbital periods because of the orbital decay of the
orbit of the inspiraling planet inside its host star. It is notewor-
thy here that all of the five Earth-sized planets that are suspected
around KIC 05807616 and KIC 10001893 have orbital periods of
only a few hours (Charpinet et al. 2011; Silvotti et al. 2014), and

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A&A 650, A205 (2021)

0.5 1.0 1.5 2.0
Planet size (R )

101

102

SN
R

HD 149382

0.4 R

0.5 1.0 1.5 2.0
Planet size (R )

100

101

CW83-1419-09

0.7 R

0.5 1.0 1.5 2.0
Planet size (R )

100

101

TYC 982-614-1

0.8 R

0.5 1.0 1.5 2.0
Planet size (R )

100

101

TYC 499-2297-1

0.9 R

SNR=10
SNR=5
0.15 R
0.16 R
0.17 R
0.18 R
0.19 R
0.20 R

Fig. 6. Performances of CHEOPS on hot subdwarfs, assuming a single 20-min transit. From left to right: HD 149382 (G = 8.9), CW83-1419-09
(G = 12.0), TYC 982-614-1 (G = 12.2), and TYC 499-2297-1 (G = 12.6). The minimum planet size for an S/N = 5 and a 0.18 R� host is indicated
next to the vertical red line.

0

20

40

60

80

100

Ph
as

e 
Co

ve
ra

ge
 (%

) HD 149382
Period: 1.0 d -> Phase Coverage: 62.3 % 
Period: 2.0 d -> Phase Coverage: 33.3 % 
Period: 3.0 d -> Phase Coverage: 23.2 % 
~80% of coverage reached at: ~0.47 d 

0

20

40

60

80

100

Ph
as

e 
Co

ve
ra

ge
 (%

) CW83-1419-09
Period: 1.0 d -> Phase Coverage: 46.0 % 
Period: 2.0 d -> Phase Coverage: 25.6 % 
Period: 3.0 d -> Phase Coverage: 17.5 % 
~80% of coverage reached at: ~0.39 d 

0

20

40

60

80

100

Ph
as

e 
Co

ve
ra

ge
 (%

) TYC 982-614-1
Period: 1.0 d -> Phase Coverage: 64.4 % 
Period: 2.0 d -> Phase Coverage: 37.4 % 
Period: 3.0 d -> Phase Coverage: 22.0 % 
~80% of coverage reached at: ~0.68 d 

0 1 2 3 4 5
Period (days)

0

20

40

60

80

100

Ph
as

e 
Co

ve
ra

ge
 (%

) TYC 499-2297-1
Period: 1.0 d -> Phase Coverage: 55.6 % 
Period: 2.0 d -> Phase Coverage: 34.3 % 
Period: 3.0 d -> Phase Coverage: 19.8 % 
~80% of coverage reached at: ~0.54 d 

Fig. 7. Phase coverage (in percent) as a function of orbital period
reached after one season of observations with CHEOPS. From top to
bottom panel: HD 149382 (7× 2 orbits), CW83 1419-09 (4× 3 orbits),
TYC 982-614-1 (6× 3 orbits), and TYC 499-2297-1 (6× 2 orbits). In
all cases, the blue lines represent the full range of 5000 periods we
explored, and the orange lines show the binning at each ∼1.7 h. The
orbital periods for which the phase coverages are ∼80% are marked with
dotted vertical red lines.

all known sdB+red dwarf or brown dwarf post-CE binaries have
orbital periods below 1 d (Schaffenroth et al. 2018, 2019, 2021).
Finally, CHEOPS provides an excellent opportunity of observing
very promising targets, such as HD 149382, which have not been
observed by Kepler, K2, or TESS.

6. Conclusions and future work

This paper presented our project that searches for transiting plan-
ets around hot subdwarfs. While no such planetary transit have
been found to date, high-quality photometric light curves are
now available for thousands of hot subdwarfs from the Kepler,
K2, TESS, and CHEOPS space missions (the harvest is continu-
ing for these last two missions). By having experienced extreme
mass loss on the RGB, these small stars (0.1–0.3 R�) constitute
excellent targets based on which the question of the evolution of
planetary systems directly after the first-ascent red giant branch
can be addressed. Hot subdwarfs also offer the potential of obser-
vationally constraining the existence of planetary remnants, that
is, planets that would have survived (even partially as a small,
possibly disintegrating, very close object) being engulfed in the
envelope of their red giant host star. Not only does the small star
size enable the detection of small remnant objects, but the ejec-
tion of the envelope may even be the reason of the survival of
such remnants by stopping the spiraling-in inside the host star.
Hot subdwarfs may therefore offer the outstanding opportunity
to study the interior of giant planets, whose exact structure is
uncertain, even for Jupiter (Wahl et al. 2017, and refererences
therein).

We first listed the hot subdwarfs observed by Kepler, K2,
TESS, and CHEOPS. We then performed injection-and-recovery
tests for a selection of representative targets from Kepler, K2,
and TESS, with the aim to determine which transiting bodies
in terms of object radius and orbital period we will be able to
detect in these light curves with our tools. For CHEOPS tar-
gets, given the filler nature of the observations (they are carried
out when CHEOPS has no time-constrained or higher-priority
observations), we directly estimated the minimum planet size
detectable from the S/N of the light curves, and then computed
the orbital periods that are covered for a given phase coverage.
For comparison purposes, we considered the same host star in all
cases.

Objects smaller than ∼1 R⊕ can be detected (if existing and
transiting) for the shortest orbital periods (about 1 d and below)
in most of the Kepler, K2, and CHEOPS targets. Values com-
parable to those for our Moon (∼0.3 R⊕) can be achieved in the
best cases. This performance of reaching sub-Earth-sized objects
is obtained only for the very few brightest TESS data, as well as
for stars with G . 13 that are observed for a significant number
(&6) of sectors. Altogether, we estimate that we will be able to
detect planets smaller than the Earth for about 250 targets for
orbital periods shorter than 1 d, if they exist. Given the rela-
tively high probability of transits for very close objects (≈5% at
1 d orbital period), our results demonstrate that we will be able

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V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

to observationally determine whether planets are able to survive
being engulfed in the envelope of their host star. Hot subdwarfs
represent a short phase of stellar evolution (∼150 Myr for the
core-He burning, i.e., EHB, phase, and about 10% of that time
for post-EHB evolution; Heber 2016), which renders the forma-
tion of second-generation planets unlikely, in particular in light
of the harsh environment for planet formation around a hot sub-
dwarf. Migration of bodies at greater distances that were not
engulfed in the envelope of the red giant host would be possi-
ble for the oldest hot subdwarfs (Mustill et al. 2018), although
their lifetime is likely too short for a complete circularization of
the orbit. Dedicated computations will be required, as those car-
ried out for the planets and remnants discovered around white
dwarfs (Veras & Fuller 2020, and references therein).

Our tests also provided a series of representative results for
the detection of larger planets at greater distances. TESS targets
will provide the most important cohort for the final goal of this
project, which is to provide statistically significant occurrence
rates of planets as a function of object radius and orbital period
around hot subdwarfs.

Our main pipeline for the search for transit events around hot
subdwarfs, SHERLOCK, has already been successfully applied
in a number of cases (Pozuelos et al. 2020; Demory et al.
2020). However, several implementations are being developed
that are especially relevant given the nature of our targets.
The first improvement involves more efficient detrending for
pulsating stars (see, e.g., Sowicka et al. 2017), in particular, high-
frequency p-mode hot subdwarf pulsators, which have relatively
high amplitudes that can hinder the detection of shallow transits.
Second, we include in SHERLOCK a model for comet-like tails
of disintegrating exoplanets, which highly differ from the typ-
ical shape of transiting exoplanets (see, e.g., Brogi et al. 2012;
Rappaport et al. 2012; Sanchis-Ojeda et al. 2015; Kennedy et al.
2019).

When a transit event in the light curves is identified that suc-
cessfully passes all the thresholds and the vetting process, we
will need to confirm the signal and associate it with a planetary
nature by scheduling follow-up observations. In order to confirm
transit events in light curves, we will trigger observations with
our Liège TRAPPIST network (Jehin et al. 2011; Gillon et al.
2011) for the deepest signals (&2500 ppm), which consists of
two 0.6 m telescopes at the La Silla (Chile) and Oukaïmeden
(Morocco) observatories. For shallower transits, we will directly
use CHEOPS, provided the target is sufficiently well visible from
the orbit of CHEOPS. When the transits are confirmed, a stellar,
white dwarf, or brown dwarf origin will need to be ruled out
based on RV measurements. We will first search for RV data
in archives that are open to the community (such as the ESO
archives) or within the hot subdwarf community. We will write
proposals for appropriate spectrographs when necessary.

Finally, we will compute the occurrence rates of planets
around hot subdwarfs by following a method similar to that of
van Sluijs & Van Eylen (2018); Wilson et al. (2019). By compar-
ing our results to these statistics for white dwarfs, to those for
∼0.8–2.3 M� main-sequence stars that are the main progenitors
of hot subdwarfs (e.g., Mayor et al. 2011; Howard et al. 2012;
Fressin et al. 2013), as well as for subgiants and RGB stars (Sato
et al. 2008; Döllinger et al. 2009; Jones et al. 2021), we will
be able to appreciate the effect of the RGB phase alone on the
evolution of exoplanetary systems.

Acknowledgements. We thank the anonymous referee for comments that
improved the manuscript. The authors thank the Belgian Federal Science Policy
Office (BELSPO) for the provision of financial support in the framework
of the PRODEX Programme of the European Space Agency (ESA) under

contract number PEA 4000131343. This work has been supported by the
University of Liège through an ARC grant for Concerted Research Actions
financed by the Wallonia-Brussels Federation. The authors acknowledge support
from the Swiss NCCR PlanetS and the Swiss National Science Foundation.
V.V.G. is a F.R.S.-FNRS Research Associate. M.G. is an F.R.S.-FNRS Senior
Research Associate. St.C. acknowledges financial support from the Centre
National d’Études Spatiales (CNES, France) and from the Agence Nationale
de la Recherche (ANR, France) under grant ANR-17-CE31-0018. K.G.I. is
the ESA CHEOPS Project Scientist and is responsible for the ESA CHEOPS
Guest Observers Programme. She does not participate in, or contribute to,
the definition of the Guaranteed Time Programme of the CHEOPS mission
through which observations described in this paper have been taken, nor to
any aspect of target selection for the programme. D.E. has received funding
from the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (project FOUR ACES;
grant agreement No 724427). This project has been carried out in the frame
of the National Centre for Competence in Research PlanetS supported by
the Swiss National Science Foundation (SNSF). G.B. acknowledges support
from CHEOPS ASI-INAF agreement n. 2019-29-HH.0. A.J.M. acknowledges
funding from the Swedish Research Council (starting grant 2017-04945) and
the Swedish National Space Agency (career grant 120/19C). A.C.C. and T.G.W.
acknowledge support from STFC consolidated grant number ST/M001296/1.
A.B. was supported by the SNSA. M.F. gratefully acknowledge the support of
the Swedish National Space Agency (DNR 65/19, 174/18). S.H. acknowledges
CNES funding through the grant 837319. S.C.C.B. acknowledges support from
FCT through FCT contracts nr. IF/01312/2014/CP1215/CT0004. S.G.S. acknowl-
edge support from FCT through FCT contract nr. CEECIND/00826/2018
and POPH/FSE (EC). This work was supported by FCT - Fundação para
a Ciência e a Tecnologia through national funds and by FEDER through
COMPETE2020 - Programa Operacional Competitividade e Interna-
cionalização by these grants: UID/FIS/04434/2019; UIDB/04434/2020;
UIDP/04434/2020; PTDC/FIS-AST/32113/2017 & POCI-01-0145-FEDER-
032113; PTDC/FIS-AST/28953/2017 & POCI-01-0145-FEDER-028953;
PTDC/FIS-AST/28987/2017 & POCI-01-0145-FEDER-028987. O.D.S.D. is
supported in the form of work contract (DL 57/2016/CP1364/CT0004) funded
by national funds through FCT. B.-O.D. acknowledges support from the
Swiss National Science Foundation (PP00P2-190080). B.N.B. acknowledges
funding through the TESS Guest Investigator Program Grant 80NSSC21K0364.
We acknowledge support from the Spanish Ministry of Science and Inno-
vation and the European Regional Development Fund through grants
ESP2016-80435-C2-1-R, ESP2016-80435-C2-2-R, PGC2018-098153-B-C33,
PGC2018-098153-B-C31, ESP2017-87676-C5-1-R, MDM-2017-0737 Unidad
de Excelencia “María de Maeztu”- Centro de Astrobiología (INTA-CSIC), as
well as the support of the Generalitat de Catalunya/CERCA programme. The
MOC activities have been supported by the ESA contract No. 4000124370. I.R.
acknowledges support from the Spanish Ministry of Science and Innovation and
the European Regional Development Fund through grant PGC2018-098153-B-
C33, as well as the support of the Generalitat de Catalunya/CERCA programme.
X.B., Se.C., D.G., M.F. and J.L. acknowledge their role as ESA-appointed
CHEOPS science team members. D.G. gratefully acknowledges financial sup-
port from the CRT foundation under Grant No. 2018.2323 “Gaseous or rocky?
Unveiling the nature of small worlds”. P.F.L.M. acknowledges support from
STFC research grant number ST/M001040/1. This project has been supported by
the Hungarian National Research, Development and Innovation Office (NKFIH)
grants GINOP-2.3.2-15-2016-00003, K-119517, K-125015, and the City of
Szombathely under Agreement No. 67.177-21/2016. This paper includes data
collected by the TESS mission. Funding for the TESS mission is provided by the
NASA Explorer Program. Funding for the TESS Asteroseismic Science Oper-
ations Centre is provided by the Danish National Research Foundation (Grant
agreement no.: DNRF106), ESA PRODEX (PEA 4000119301) and Stellar Astro-
physics Centre (SAC) at Aarhus University. We thank the TESS team and staff
and TASC/TASOC for their support of the present work. This work has made use
of data from the ESA mission Gaia (https://www.cosmos.esa.int/gaia),
processed by the Gaia Data Processing and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding
for the DPAC has been provided by national institutions, in particular the
institutions participating in the Gaia Multilateral Agreement.

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1 Space sciences, Technologies and Astrophysics Research (STAR)
Institute, Université de Liège, 19C Allée du 6 Août, 4000 Liège,
Belgium
e-mail: valerie.vangrootel@uliege.be

2 Astrobiology Research Unit, Université de Liège, Allée du 6 Août
19C, 4000 Liège, Belgium

3 Institut de Recherche en Astrophysique et Planétologie, CNRS,
Université de Toulouse, CNES, 14 avenue Edouard Belin,
31400 Toulouse, France

4 Observatoire Astronomique de l’Université de Genève, Chemin
Pegasi 51, Versoix, Switzerland

5 Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6,
3012 Bern, Switzerland

6 Center for Space and Habitability, Gesellsschaftstrasse 6,
3012 Bern, Switzerland

7 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
8 Instituto de Astrofísica e Ciências do Espaço, Universidade do

Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
9 Department of Physics, High Point University, One University

Parkway, High Point, NC 27268, USA
10 Universidad Internacional de Valencia (VIU), Carrer del Pintor

Sorolla 21, 46002 Valencia, Spain

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mailto:valerie.vangrootel@uliege.be


V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

11 Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071
Granada, Spain

12 Department of Physics, Astronomy and Materials Science, Missouri
State University, 901 S. National, Springfield, MO 65897, USA

13 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife,
Spain

14 Departamento de Astrofísica, Universidad de La Laguna, 38206 La
Laguna, Tenerife, Spain

15 Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, Can
Magrans s/n, 08193 Bellaterra, Spain

16 Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona,
Spain

17 ESTEC, European Space Agency, 2201AZ, Noordwijk, NL, The
Netherlands

18 Depto. de Astrofísica, Centro de Astrobiologia (CSIC-INTA), ESAC
campus, 28692 Villanueva de la Cãda (Madrid), Spain

19 Departamento de Física e Astronomia, Faculdade de Ciências, Uni-
versidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

20 Space Research Institute, Austrian Academy of Sciences, Schmiedl-
strasse 6, 8042 Graz, Austria

21 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
22 Department of Astronomy, Stockholm University, AlbaNova Univer-

sity Center, 10691 Stockholm, Sweden
23 INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123

Catania, Italy
24 ADMATIS, 3534 Miskolc, Kandó Kálmán u. 5., Hungary
25 Institute of Planetary Research, German Aerospace Center (DLR),

Rutherfordstrasse 2, 12489 Berlin, Germany
26 Centre for Exoplanet Science, SUPA School of Physics and Astron-

omy, University of St Andrews, North Haugh, St Andrews KY16
9SS, UK

27 Université de Paris, Institut de physique du globe de Paris, CNRS,
75005 Paris, France

28 Lund Observatory, Dept. of Astronomy and Theoretical Physics,
Lund University, Box 43, 22100 Lund, Sweden

29 Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA
Leiden, The Netherlands

30 Department of Space, Earth and Environment, Chalmers University
of Technology, Onsala Space Observatory, 43992 Onsala, Sweden

31 Dipartimento di Fisica, Università degli Studi di Torino, via Pietro
Giuria 1, 10125 Torino, Italy

32 University of Vienna, Department of Astrophysics, Türkenschanzs-
trasse 17, 1180 Vienna, Austria

33 Department of Physics, University of Warwick, Gibbet Hill Road,
Coventry CV4 7AL, UK

34 Konkoly Observatory, Research Centre for Astronomy and Earth
Sciences, 1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary

35 ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter
sétány 1/A, 1117 Budapest, Hungary

36 Sydney Institute for Astronomy, School of Physics A29, University
of Sydney, NSW 2006, Australia

37 IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sor-
bonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France

38 Institut d’astrophysique de Paris, UMR7095 CNRS, Université
Pierre & Marie Curie, 98bis blvd. Arago, 75014 Paris, France

39 INAF, Osservatorio Astronomico di Padova, Vicolo
dell’Osservatorio 5, 35122 Padova, Italy

40 Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
41 Institute of Optical Sensor Systems, German Aerospace Center

(DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
42 Dipartimento di Fisica e Astronomia, Università degli Studi di

Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
43 Centre Spatial de Liège, STAR institute, Université de Liège, avenue

du Pré Aily, 4031 Angleur (Liège), Belgium
44 Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE,

UK
45 Center for Astronomy and Astrophysics, Technical University

Berlin, Hardenberstrasse 36, 10623 Berlin, Germany
46 Institut für Geologische Wissenschaften, Freie Universität Berlin,

12249 Berlin, Germany
47 INAF – Osservatorio Astrofisico di Torino, Strada dell’Osservatorio

20, 10025 Pino Torinese, Italy
48 ELTE Eötvös Loránd University, Gothard Astrophysical Observa-

tory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
49 MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent

Imre h. u. 112, Hungary
50 Institute of Astronomy, University of Cambridge, Madingley Road,

Cambridge, CB3 0HA, UK

A205, page 15 of 19



A&A 650, A205 (2021)

Appendix A: Hot subdwarfs observed in the original Kepler field

Table A.1. Hot subdwarfs observed in the original Kepler field.

KIC Class Other name Kp Quarters (SC) Quarters (LC)
sdB pulsators
9 472 174 sdB+dM 2M1938+4603 12.3 Q0, Q5-Q17.2 All: Q0-Q17.2
2 437 937 sdB B5 (NGC6791) 13.9 Q11.X Q11
3 527 751 sdB J19036+3836 14.8 Q2, Q5-Q17.2 Idem SC
11 558 725 sdB+WD J19265+4930 14.9 Q3.3, Q6-Q17.2 Q3, Q5-Q17.2
5 807 616 sdB KPD 1943+4058 15.0 Q2.3, Q5-Q17.2 Idem SC
10 553 698 sdB J19531+4743 15.1 Q4.1, Q8-Q10, Q12-Q14, Q16-Q17.2 Q4-Q6, Q8-Q10, Q12-Q17.2
2 697 388 sdB J19091+3756 15.4 Q2.3, Q5-Q17.2 Idem SC
7 668 647 sdB+WD FBS1903+432 15.4 Q3.1, Q6-Q17.2 Q3.1, Q5-Q17.2
10 001 893 sdB J19095+4659 15.8 Q3.2, Q6-Q17.2 Q3.2, Q5-Q17.2
10 139 564 sdB J19249+4707 16.1 Q2.1, Q5-Q17.2 Idem SC
8 302 197 sdB J19310+4413 16.4 Q3.1, Q5-Q17.2 except Q12 idem SC
7 664 467 sdB J18561+4319 16.4 Q2.3, Q5-Q17.2 except Q12 idem SC
10 670 103 sdB J19346+4758 16.5 Q2.3, Q5-Q17.2 Idem SC
11 179 657 sdB+dM J19023+4850 17.1 Q2.3, Q5-Q17.2 except Q8 and Q12 idem SC
2 991 403 sdB+dM J19272+3808 17.1 Q1, Q5-Q17.2 Idem SC
2 991 276 sdB J19271+3810 17.4 Q2.1, Q6-Q17.2 except Q12 Idem SC
2 569 576 sdB B3 (NGC6791) 18.1 Q11.3, Q14-Q17.2 Q11, Q14-Q17.2
2 438 324 sdB+dM B4 (NGC6791) 18.3 Q6-Q17.2 Idem SC
sdB/sdOB non pulsators
6 848 529 sdB+? BD +42 3250 10.7 Q0 All: Q0-Q17.2
1 868 650 sdB+dM KBS 13 13.4 Q1 All: Q0-Q17.2
9 543 660 sdOB 13.8 Q1 Q1-Q17, except Q7 and Q11
10 982 905 sdB+F/G J19405+4827 14.1 Q2.1 Q2-Q10
6 188 286 sdOB 14.2 Q2.3 Q2, Q6-Q8, Q14-Q16
8 054 179 He-sdOB 14.4 Q3.1, Q6 Q3.1, Q4-Q17.2 except Q11 and Q12
7 975 824 sdOB+WD KPD 1946+4340 14.6 Q1, Q5-Q12 Q1, Q5-Q17.2
10 449 976 He-sdOB 14.9 Q3.2 Q3, Q5-Q9
3 353 239 sdB 15.2 Q4.1 Q4-Q5, Q7-Q9, Q13-Q17
10 593 239 sdB+F/G J19162+4749 15.3 Q2.3 Q2, Q5-Q17.2
2 569 583 sdB B6 (NGC6791) 15.4 Q11.2 Q11
7 104 168 sdB 15.5 Q3.1 Q3, Q5-Q9
10 149 211 sdB+? 15.5 Q4.2 Q4-Q17.2
10 789 011 sdOB 15.5 Q3.2 Q3, Q5-Q10
11 350 152 sdB+F/G 15.5 Q3.1 Q3, Q5-Q10
7 434 250 sdB+? J19135+4302 15.5 Q2.3 Q2, Q5-Q17.2
2 020 175 sdB 15.5 Q3.1 Q3, Q5-Q10, Q13-Q17.2
12 021 724 sdB+WD? 15.6 Q4.2 Q4-Q10
3 343 613 He-sdOB 15.7 Q3.2 Q3, Q5-Q10
5 938 349 sdB 16.1 Q3.2 Q3, Q10
6 614 501 sdB+WD? 16.1 Q3.3, Q5, Q6, Q8-Q10 Q3.3, Q5-Q17.2
9 211 123 sdB 16.1 Q3.3 Q3, Q5-Q10, Q13-Q17.2
9 957 741 He-sdOB 16.1 Q2.1 Q2, Q6-Q9
2 304 943 sdB 16.2 Q3.3 Q3, Q10
8 496 196 sdOB 16.4 Q2.3 Q2, Q6-Q10
8 874 184 sdB+? 16.5 Q4.1 Q4-Q10, Q13-Q17.2
8 022 110 sdB 16.5 Q2.3 Q2, Q6-Q10, Q13-Q17.2
6 878 288 He-sdOB+? 16.7 Q3.1 Q3, Q5-Q10
6 522 967 sdB 16.9 Q3.2 Q3, Q10
7 799 884 sdB 16.9 Q4.1 Q4.1
10 462 707 sdB+WD? 16.9 Q4.1 Q4.1, Q10
11 400 959 sdB 16.9 Q4.1 Q4.1
10 784 623 sdB 17.0 Q10 Q4-Q10 except Q8
10 961 070 sdOB 17.0 Q4.2 Q4.2
3 527 028 sdB+? 17.1 Q4.2 Q4-Q10
5 340 370 sdB+? 17.1 Q4.2 Q4, Q10
9 569 458 sdB 17.2 Q1 Q1
8 889 318 sdB 17.2 Q2.3 Q2.3, Q13-Q17.2
9 408 967 He-sdOB 17.2 Q2.3 Q2.3, Q10
4 244 427 sdB 17.3 Q2.1, Q6-Q10 Q2.1, Q6-Q17.2 except Q12
8 142 623 sdB+? J18427+4404 17.3 Q1 Q1, Q5-Q17.2
11 357 853 sdOB 17.4 Q2.1 Q2.1
3 527 617 He-sdOB 17.5 Q2.2 Q2.2
3 729 024 sdB 17.6 Q2.2 Q2.2
9 095 594 sdB 17.7 Q3.2 Q3.2
5 342 213 sdOB 17.7 Q2.2 Q2.2, Q14-Q16
10 661 778 sdB 17.7 Q2.3, Q6-Q10 Q2.3, Q6-Q17.2 except Q11 and Q12
sdO non pulsators
7 755 741 sdO 13.7 Q1 Q1-Q17
9 822 180 sdO+F/G 14.6 Q2.1, Q6 Q2.1, Q6-Q10
7 353 409 sdO 14.7 Q2.2, Q5 Q2.2, Q5-Q9
10 207 025 He-sdO 15.0 Q3.3 Q3.3, Q5-Q9
7 335 517 sdO+dM 15.7 Q3.2, Q6 Q3.2, Q5-Q17.2
2 297 488 sdO+F/G 17.2 Q1 Q1
2 303 576 He-sdO+? 17.4 Q3.3, Q6 Q3.3, Q6-Q17.2

Notes. Commissioning (9.7 days starting 2 May 2009): Q0; survey phase: Q1: 33.5 d (12 May–14 June 2009), Q2, Q3, and Q4: about 90 days
each, divided in 3, i.e., monthly surveys; rest of the mission: Q5–Q16: about 90 days each; mission stopped at Q17.2 (11 May 2013).

A205, page 16 of 19



V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

Appendix B: List of hot subdwarfs observed in the K2 fields

Table B.1. Hot subdwarfs observed in the K2 fields.

KIC Class Other name Kp Campaign (SC) Campaign (LC)

sdB pulsators
220 641 886 sdB HD 4539 10.40 8 8
228 755 638 sdB+dM HW Vir 10.76 10 (101-102) 10 (101-102)
211 623 711 He-sdB UVO 0825+15 11.89 5;18 5;18
220 376 019 sdB+WD PG 0101+039 12.11 8 8
220 422 705 sdB+G PG 0039+049 12.87 8 8
249 942 493 sdB EC 15 103-1557 12.89 15 15
211 779 126 sdB 2M0856+1701 12.92 5;18 5;18
246 387 816 sdB+dM EQ Psc 12.92 12 12
246 023 959 sdB+dM PHL 457 13.04 12 12
211 881 419 iHe-sdB PG 0848+186 13.30 16;18 5;16;18
201 203 416 sdB PG 1156-037 13.46 10 (101-102) 10 (101-102)
248 411 044 sdB UY Sex 13.56 14 14
246 141 920 sdB PHL 531 13.99 12 12
211 433 013 sdB+WD LT Cnc 14.02 16 16
211 765 471 sdB+WD HZ Cnc 14.04 5;16;18 5;16;18
220 614 972 sdB+F PG 0048+091 14.29 8 8
211 392 098 sdB+MS SDSS J082517.99+113106.3 14.34 18 5;18
211 437 457 sdB PG 0902+124 14.73 16 16
246 683 636 sdB+dM V1405 Ori 15.07 13 13
248 368 659 sdB+WD VPHAS J181343.0-213 843.9 15.10 9 (91-92) 9 (91-92)
212 508 753 sdB+F7 PG 1315-123 15.13 6;17 6;17
211 823 779 sdB+F1 SDSS J082003.35+173914.2 15.22 5;18 5;18
212 475 716 sdB+MS EC 13 356-1300 15.24 17 17
211 696 659 sdB+WD SDSS J083603.98+155216.4 15.50 5;18 5;18
212 707 862 sdB SDSS J135544.71-080354.3 15.55 6;17 6;17
212 204 284 sdB PG 0843+246 15.64 16 16
246 283 223 sdB HE 2307-0340 15.66 12 12
248 368 658 sdB 15.70 9 (91-92) 9 (91-92)
218 717 602 sdB 15.76 7 7
211 938 328 sdB+F6 LB 378 15.78 5;18 5;18
218 366 972 sdB+WD 15.94 7 7
201 206 621 sdB+WD PG 1142-037 15.99 1 1
212 487 276 sdB EC 13 359-1245 16.23 17 17
217 280 630 sdB 16.33 7 7
215 776 487 sdB 16.35 7 7
203 948 264 sdB 16.70 2 2
246 373 305 iHe-sdB PHL 417 16.88 12 12
251 668 197 sdB EC 15 094-1725 17.00 15 15
229 002 689 sdB SDSS J122057.48-012642.3 18.65 10 (101-102) 10 (101-102)
220 188 903 sdB+WD PB 6373 14.91 no data 8
230 195 595 sdB 15.59 no data 11

sdB/sdOB non pulsators, single
234 319 842 sdB 12.97 11 (111-112) 11 (111-112)
60 017 832 sdB PG 2349+002 13.27 T
211 708 181 sdB GALEX J081233.6+160121 13.77 5 5
227 389 858 sdB 13.79 11 (111-112) 11 (111-112)
246 230 928 sdB PHL 529 13.93 12 12
206 535 752 sdB PHL 358 13.99 3 3
201 648 341 sdB PG 1214+031 14.04 10 (101-102) 10 (101-102)
217 204 898 sdB 14.26 7 7
246 643 895 sdB HS 0446+1344 14.50 13 13
212 722 777 sdB PG 1330-074 14.93 17 17
211 727 748 sdB PG 0838+165 14.99 5;16 5;16
206 073 023 sdB BPS CS 29 512-38 15.00 3 3
210 837 690 sdB 15.11 4 4
212 498 842 sdB EC 13 162-1229 15.26 6 6
212 465 180 sdB EC 13 265-1313 15.56 6 6

Notes. T: engineering test from 4 to 13 February 2014; Campaign 0 (8 March–27 May 2014) to 18 (12 May–2 July 2018), https://keplergo.
github.io/KeplerScienceWebsite/k2-fields.html

A205, page 17 of 19

https://keplergo.github.io/KeplerScienceWebsite/k2-fields.html
https://keplergo.github.io/KeplerScienceWebsite/k2-fields.html


A&A 650, A205 (2021)

Table B.1. continued.

KIC Class Other name Kp Campaign (SC) Campaign (LC)

212 160 066 sdB SDSS J082445.68+231520.3 15.57 18 5;18
246 901 153 sdB KUV 04369+1640 15.70 13 13
249 601 610 sdB EC 15 050-2017 15.71 15 15
246 980 092 sdB KUV 04482+1727 15.74 13 13
218 148 570 sdB 15.74 7 7
228 914 323 sdB PG 1249-028 15.76 10 (101-102) 10 (101-102)
228 682 488 sdB SDSS J085217.70+211637.4 16.00 16 16
212 818 294 sdB PG 1356-047 16.15 6;17 6;17
248 422 838 sdB PG 1032+007 16.27 14 14
214 515 136 sdB 16.30 7 7
251 603 936 sdB SDSS J131916.15-011405.0 16.69 17 17
201 531 672 sdB SDSS J112757.48+010044.2 16.89 1 1
251 457 058 sdB SDSS J105428.85+010514.7 17.10 14 14
246 371 369 sdB PB 5212 17.11 12 12
211 552 072 sdB SDSS J084556.85+135211.3 17.50 16 16
212 567 176 sdB HE 1309-1102 17.65 6 6
249 585 191 sdB EC 15 064-2029 17.95 15 15
248 840 987 sdB SDSS J102050.99+114024.3 18.15 14 14
248 810 568 sdOB SDSS J110055.94+105542.3 14.22 14 14
246 997 679 sdOB KUV 05109+1739 14.58 13 13
211 421 561 sdOB SDSS J090042.68+115749.9 14.90 16 16
220 265 912 sdOB PG 0055+016 15.19 8 8
249 700 050 sdOB EC 15 059-1902 15.65 15 15
206 240 954 sdOB SDSS J220337.88-090733.5 16.31 3 3
210 731 139 sdOB SDSS J032427.24+184918.2 16.37 4 4
246 087 406 sdOB PB 7470 16.46 12 12
206 186 190 sdOB BPS CS 22 886-65 16.49 3 3
251 605 347 sdOB SDSS J133611.02-011156.0 18.69 17 17
246 745 570 He-sdB KUV 04456+1502 15.68 13 13
211 920 209 He-sdB PG 0850+192 16.39 18 5; 16; 18
249 770 424 He-sdOB GALEX J152332.2-181 726 14.00 15 15
211 495 446 He-sdOB PG 0838+133 14.03 5;16 5;16
248 748 173 He-sdOB PG 1033+097 16.38 14 14
248 761 152 He-sdOB PG 1045+100 17.09 14 14
248 915 544 He-sdOB SDSS J103806.64+134412.1 17.21 14 14
211 841 249 sdB SDSSJ082734.96+175356.0 14.64 5;18 5;18
250 083 298 sdB EC15203-1418 17.34 15 15

sdB/sdOB non pulsators, in binaries
220 468 352 sdB+F PB 6355 13.01 8 8
251 377 113 sdB+F/G SDSS J090827.24+231417.9 13.53 16 16
211 499 370 sdB+F/G/K SDSS J082556.80+130753.5 14.60 5 5;18
218 637 228 sdB+F/G 14.79 7 7
227 441 033 sdB+F/G 15.10 11 (111-112) 11 (111-112)
216 924 452 sdB+F/G 15.53 7 7
250 121 838 sdB+F/G/K EC 15 365-1350 15.74 15 15
246 151 922 sdB+G9 HE 2322-0617 15.74 12;19 12;19
212 630 158 sdB+F/G 15.75 6 6
246 868 556 sdB+F/G GALEX J050252.2+162647 15.78 13 13
246 864 591 sdB+F/G/K KUV 04571+1620 15.98 13 13
211 910 684 sdB+F/G PG 0906+191 15.99 16 16
212 108 396 sdB+F/G SDSS J082447.30+221112.9 16.02 5 5;18
211 400 847 sdB+F/G SDSS J084447.00+113910.0 16.43 5 5;18
212 003 762 sdB+F/G SDSS J081406.79+201901.7 16.51 18 18
212 137 838 sdB+F/G Ton 920 16.54 5 5
250 152 590 sdB+F/G/K LB 889 17.13 15 15
248 467 942 sdB+F/G SDSS J103022.07+020524.3 17.24 14 14
211 732 575 sdB+F/G SDSS J082426.51+162145.1 17.68 18 18
251 583 165 sdB+F/G SDSS J131932.19-014131.2 18.24 17 17
212 866 280 sdB+F/G SDSS J133701.51-031732.2 18.27 17 17
212 410 755 sdB+WD EC 13 332-1424 13.46 6 6
201 535 046 sdB+? PG 1049+013 14.44 14 14
251 372 905 sdOB+F/G SDSS J091216.06+225452.7 15.30 16 16
211 904 152 sdOB+F/G PG 0912+189 15.93 16 16
248 767 552 sdOB+WD? SDSS J101833.11+095336.1 14.97 14 14
246 877 984 sdOB+WD KUV 05053+1628 16.11 13 13

A205, page 18 of 19



V. Van Grootel et al.: A search for transiting planets around hot subdwarfs. I.

Table B.1. continued.

KIC Class Other name Kp Campaign (SC) Campaign (LC)

sdO non pulsators
212 762 631 sdO PG 1355-064 13.76 6 6;17
220 179 214 sdO GD 934 14.93 8 8
248 520 995 sdO SDSS J110053.55+034622.8 17.25 14 14
211 517 387 sdO SDSS J082944.74+132302.5 17.32 5 5
249 862 817 sdO EC 15 447-1656 18.05 15 15
228 821 386 He-sdO PG 1220-056 14.86 10 (101-102) 10 (101-102)
249 867 379 He-sdO EC 15 348-1652 15.35 15 15
205 247 324 He-sdO 16.01 2 2
201 640 895 He-sdO SDSS J110215.45+024034.2 17.60 14 14
228 960 704 He-sdO SDSS J123821.48-021211.4 18.49 10 (101-102) 10 (101-102)

Misc., in LC only
201 150 341 sdB HE 1140-0500 14.50 no data 1
214 958 569 sdB 15.70 no data 7
216 775 790 sdB 16.50 no data 7
201 236 182 sdB PG 1154-031 16.59 no data 1
211 720 816 sdB SDSS J083901.50+161148.0 16.71 no data 5; 16; 18
211 594 465 sdB SDSS J081931.22+142756.1 17.19 no data 5; 18
248 912 731 sdB SDSS J103832.41+133848.3 17.44 no data 14
201 201 339 sdB SDSS J112757.48+010044.2 17.50 no data 1
201 590 024 sdB SDSS J113418.00+015322.1 17.65 no data 1
201 698 091 sdB SDSS J114821.29+033625.7 17.70 no data 1
229 021 782 sdB SDSS J125410.86-010408.3 17.72 no data 10
228 682 339 sdB SDSS J082824.20+212556.7 17.73 no data 5; 16; 18
251 457 060 sdB SDSS J104725.10+010847.2 17.80 no data 14
248 783 069 sdB SDSS J104620.14+101629.7 18.65 no data 14
251 410 019 sdB SDSS J085809.09+252134.6 18.87 no data 16
201 424 163 sdB+WD PG 1136-003 15.96 no data 1
228 682 347 sdB+WD SDSS J083139.68+162316.4 17.91 no data 5
248 783 744 sdB+WD SDSS J103218.40+101725.8 18.82 no data 14
211 460 944 sdB+WD ? SDSS J084556.85+135211.3 15.36 no data 16
228 796 212 sdB SDSS J124446.64-065625.8 18.83 no data 10
211 991 114 sd+F/G Ton 914 15.10 no data 5; 18
211 930 840 He-sdB SDSS J091512.06+191114.6 19.13 no data 16
201 734 164 sdOA PG 1110+045 14.84 no data 1
213 545 287 sdOB GALEX J191509.0-290 311 15.00 no data 7
201 924 421 sdOB SDSS J113218.41+075103.0 17.20 no data 1
228 682 323 sdOB SDSS J082110.89+183924.1 17.84 no data 5
212 034 957 sdOB SDSS J090302.39+205008.9 18.62 no data 16
215 669 184 He-sdOB GALEX J193323.6-234 553 15.00 no data 7
201 802 867 He-sdOB SDSS J111633.29+052507.9 17.80 no data 1
251 383 153 He-sdOB SDSS J091044.90+234044.6 18.27 no data 16
229 155 531 He-sdOB SDSS J121643.72+020835.9 18.73 no data 10
251 357 585 He-sdOB SDSS J092245.79+214238.9 19.01 no data 16
211 602 914 sd SDSS J082959.28+143441.8 15.64 no data 5; 18
213 716 821 sdO GALEX J192041.4-282 939 13.40 no data 7
246 735 349 He-sdO KUV 04402+1455 13.97 no data 13
214 453 765 sdO GALEX J191158.1-262 712 15.30 no data 7
216 452 306 He-sdO 16.40 no data 7
231 422 890 sdO 17.07 no data 11
201 418 759 sdO SDSS J111438.57-004024.3 18.10 no data 1
201 843 731 sdO SDSS J115009.48+061042.1 18.10 no data 1
211 559 083 sdO SDSS J084421.10+135807.6 18.18 no data 16
228 682 365 He-sdO SDSS J083747.23+194955.9 18.60 no data 5
216 747 137 sdO+dM 2MASS J18521800-2 147 506 13.87 no data 7
217 750 936 sdO+dM 16.70 no data 7

Appendix C: List of hot subdwarfs observed in the
TESS primary mission

That is, Sector 1–26. The list is available at https://github.
com/franpoz/Hot-Subdwarfs-Catalogues or at the CDS.

Appendix D: List of hot subdwarfs observed in the
TESS extended mission

Sector 27–32. The list is available at https://github.com/
franpoz/Hot-Subdwarfs-Catalogues or at the CDS.

A205, page 19 of 19

https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/Hot-Subdwarfs-Catalogues
https://github.com/franpoz/Hot-Subdwarfs-Catalogues

	A search for transiting planets around hot subdwarfs
	1 Introduction
	2 Search for planets around hot subdwarfs: current status
	3 Observations and methods
	3.1 Space-based light curves of hot subdwarfs
	3.2 Tools for transit searches in space-based light curves

	4 Injection-and-recovery tests
	4.1 Results for Kepler and K2
	4.2 Results for TESS

	5 CHEOPS performances for hot subdwarfs
	6 Conclusions and future work
	Acknowledgements
	References
	Appendix A: Hot subdwarfs observed in the original Kepler field
	Appendix B: List of hot subdwarfs observed in the K2 fields
	Appendix C: List of hot subdwarfs observed in the TESS primary mission
	Appendix D: List of hot subdwarfs observed in the TESS extended mission