Hot subluminous O stars from the SDSS

We carried out a quantitative spectral analysis of 73 hot subluminous O-stars selected from the SDSS spectral database. While the helium deficient sdOs are scattered over a wide range of effective temperature and gravity, the helium enriched sdO stars are concentrated in a small intervall of 40kK to 50kK and log g = 5.5 ... 6.0. Comparing the distribution in the T_eff-log g-diagram with evolutionary tracks, we find the helium deficient sdOs to be the progeny of the sdB stars. The results for the helium enriched ones are less conclusive. Both the merger of two white dwarfs and the delayed helium core flash scenarios are viable options to be explored further.