2017ApJ...835..131K

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period of P_orb = 87.49668(1) minutes (0.060761584(10) days), making it the second-most compact sdB binary known. The light curve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass of M_sdB=0.45_–0.07^+0.09M_☉, a companion white dwarf mass of M_WD=0.46_–0.09^+0.12M_☉, and a mass ratio of q=M_WDM_sdB=1.03_–0.08^+0.10. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of M_sdB=0.47M_☉, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 minutes at the exit from the last common envelope (CE) phase. For longer CE exit periods, the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived logg and T_eff with our MESA models, we find that an sdB model with a hydrogen envelope mass of 5×10^–4 M_☉ matches the measurements at a post-CE age of 94 Myr, corresponding to a post-CE orbital period of 109 minutes, which is close to the limit to start accretion while the sdB is still burning helium.