10.6.1 Introduction

Only a few tens of stellar black holes (BH) in short-period binaries (BHB) are known in the Galaxy. All known BHB were found by their X-ray emission, generated by mass transfer from the optical companion onto the compact object. However, most of the short-period BHB are not known because they do not emit X-rays, either because the optical star does not fill its Roche-lobe or the accretion disc is in a quiescent phase. The Compact Companion work package aims to detect candidates of dormant BHB (and dormant binaries with neutron stars or white dwarfs) by identifying optical stars that display large ellipsoidal periodic modulations, induced by tidal interaction with their compact companions. This SOS work package considered short-period variables identified by supervised classification (Section 10.3) as ellipsoidals, re-analyzed their $G$-band modulation, and identified 6306 main-sequence systems as candidates for having compact-object secondaries (see table vari_compact_companion), assuming their observed periodic modulations reflect indeed the ellipsoidal effect. Further details are presented in Gomel et al. (2023).