Abstract
We discovered that HD 188112, a bright (V= 10 m: 2), nearby (80 pc) 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 Te = 21 500 500 K, 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 188.3 km s 1 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 (0.24 M). 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 0.73 M 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 (1.4 M), 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.
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