Subluminous B Stars and Progenitors of Helium Core White Dwarfs

Abstract

Subluminous B (sdB) stars can result from stable Roche lobe overflow or common envelope ejection in close binary systems. The companions are either low mass main sequence stars or white dwarfs. We discuss mass determinations for sdB stars from such systems and report the discovery of the bright nearby sdB star, HD 188112, to be a close binary system of unusually low mass (0.23 solar masses). From the mass function it is evident that the companion is a massive compact object of at least 0.72 solar masses. The mass of the sdB HD 188112 is too low for the star to evolve into a C/O white dwarf, but it probably is a progenitor for a helium core white dwarf.