2003A&A...411L.477H

We discovered that HD 188112, a bright (V=10.2mag), nearby (80pc) B-type star, is a unique subluminous B (sdB) star. SdB stars are usually identified with models of core helium burning Extreme Horizontal-Branch (EHB) stars of half a solar mass. A spectral analysis of the hydrogen and helium lines resulted in T_eff=21500±500K, logg=5.66±0.05 placing the star below the EHB. HD 188112 was found to be radial velocity (RV) variable and the RV curve has been measured to be perfectly sinusoidal with a period of 0.606585 days and a semi-amplitude of 188.3km/s indicating that it is a close binary system. From the atmospheric parameters and the Hipparcos parallax we conclude that the sdB star is of low mass (0.24M_☉). The mass of the sdB is too low to sustain core helium burning and it is now evolving into a helium core white dwarf. A lower limit to the mass of its unseen companion of 0.73M_☉ is derived from the mass function. Because the companion does not contribute to the spectral energy distribution from the UV to the infrared it cannot be a main sequence star but must be a white dwarf (WD), a neutron star (NS) or a black hole. The system would qualify as pre-supernova Ia candidate (sdB+WD) if its total mass is above the Chandrasekhar limit (1.4M_☉), or as post-supernova (sdB+NS) if the companion mass is above that limit, requiring the inclination angle to be lower than 51° or 48°, respectively.