Subdwarf B and O stars : which evolutionary pathways?

Hot subdwarf stars (spectral types subdwarf B and O) are long lived stars producing a large amount of UV radiation. This makes them excellent candidates to explain the UV radiation observed in old populations. However, the origin of both classes of hot subdwarfs is unclear. I review possible single star and binary channels. High resolution observations of hot subdwarfs taken in the course of the Supernova type Ia Progenitor surveY (SPY) are presented. The SPY observations are used for a systematic assessment of the frequency of close binaries among hot subdwarfs. Results are a high binary fraction among the subdwarf B stars – albeit not as high as in a previous investigation, but a very low binary frequency in helium-rich hot subdwarf O stars. Implications for the evolutionary status of hot subdwarfs are discussed.