Hot subdwarfs

Hot subdwarfs can be divided broadly into subdwarf O-stars (sdO) and subdwarf-B stars (sdB). The majority of sdB stars are extreme horizontal branch (EHB) stars, i.e., core-helium burning stars with a mass close to 0.5 M-circle dot and with a very thin, inert hydrogen envelope. It has recently been discovered that most sdB stars are spectroscopic binaries with orbital periods of hours or days. I review the observed properties of these binaries and discuss the constraints these observations impose on models for the formation of the EHB stars. The sdO stars are a heterogeneous group. I highlight some recent results for binary sdO stars.