SIMBAD references

We present an analysis of two recently discovered, very hot hydrogen-deficient post-AGB stars of spectral type O(He), namely HS 1522+6615 and HS 2209+8229, by means of state-of-the-art NLTE model atmospheres based on new optical, UV, and X-ray observations. In the spectra of HS 1522+6615 we discovered a variability of the O Rco λ#3Å line complex on a time scale of few days. We also report on the discovery of a ring structure in the planetary nebula K1-27 which has an O(He)-type central star. We discuss how the group of O(He) stars fits in our picture of hydrogen-deficient post-AGB stellar evolution.