  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.
