Subdwarf O Stars from SDSS and SPY
  We present the results of quantitative spectral analysis of sdOs from the SDSS database. Fitting the Balmer, HeI and HeII lines to non-LTE model spectra, we derived their effective temperatures, surface gravities and helium abundances. We find the helium-enriched sdO stars to rally in a small intervall of Teff = 40,000K to 50,000K and log(g) = 5.5 to 6.0. We conclude, that the sdB stars are linked with the helium-deficient sdO stars, i.e. the sdBs are predecessors for the helium-deficient sdOs. To explain the helium-enriched sdO stars in their entirety, we regard two scenarios as most promising: The merging of two helium-core white dwarfs and the late hot flashers. 
H. A. Hirsch, U. Heber, S. J. O'Toole