  Hot subdwarf stars (sdO/Bs) are the stripped cores of red giants located at
the bluest extension of the horizontal branch. They constitute the dominant
population of UV-bright stars in old stellar environments and are most likely
formed by binary interactions. We perform the first systematic, spectroscopic
analysis of a sample of those stars in the Galactic halo based on data from
SDSS. In the course of this project we discovered 177 close binary candidates.
A significant fraction of the sdB binaries turned out to have close substellar
companions, which shows that brown dwarfs and planets can significantly
influence late stellar evolution. Close hot subdwarf binaries with massive
white dwarf companions on the other hand are good candidates for the
progenitors of type Ia supernovae. We discovered a hypervelocity star, which
not only turned out to be the fastest unbound star known in our Galaxy, but
also the surviving companion of such a supernova explosion.
