  Hot subluminous stars can be roughly divided into B- and O-types. Unlike the
latter many sdBs are found in close binaries, indicating that binary evolution
plays a vital role. Recent NLTE spectral analyses revealed that an evolutionary
link between sdB stars and sdO stars is plausible only for the helium-deficient
sdO stars, i.e. they are the likely successors to sdB stars. The atmospheric
properties of helium-enriched sdO stars can be explained by the late hot
flasher as well as by the white-dwarf merger scenarios, although both models do
not match the observed properties of helium-enriched sdO stars in detail. The
white dwarf merger scenario is favoured because it naturally explains the
scarcity of close binaries amongst helium-enriched sdO stars. A hyper-velocity
sdO star moving so fast that it is unbound to the Galaxy has probably been
ejected by the super-massive black hole in the Galactic centre.
