Properties of atmospheres and winds of H-deficient central stars and related objects

According to latest results presented at this workshop, it appears that the surface chemistry of hydrogen-deficient post-AGB stars can now be explaine d by evolutionary calculations involving a late He-shell flash. A quantitat ive comparison of theoretical predictions with observed element abundances can eventually constrain details of this evolutionary scenario. I will emph asize here the diverse chemical compositions observed in H-deficient post-A GB stars, which call for an explanation. The abundance patterns vary from a lmost pure helium atmospheres to hydrogen- and helium-free atmospheres, wit h a rich choice of compositions in between, involving rather different mixt ures of H, He, and CNO.