O(He) Stars
  Spectral analyses of H-deficient post-AGB stars have shown that a small group of four extremely hot objects exists which have almost pure He absorption-line spectra in the optical. These are classified as O(He) stars. For their evolution there are two scenarios: They could be the long-sought hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double-degenerate merging process. An alternative explanation might be that O(He) stars are post early-AGB stars. These depart from the AGB just before they experience their first thermal pulse (TP) which will then occur as a late thermal pulse (LTP). This would be a link to the low-mass He-enriched sdO stars and low-mass, particularly He-rich PG1159 stars. 
T. Rauch (1), E.Reiff (1), K. Werner (1), J. W. Kruk (2) ((1)   Institute for Astronomy and Astrophysics, Kepler Center for ...