SIMBAD references

The observational data for the planetary nebulae with hydrogen-deficient central stars are analysed. We show that the general evolutionary sequence is: late-[WC], early-[WC], PG1159. An analysis of the observed distributions of nebular parameters leads to a conclusion that the planetary nebulae with hydrogen-deficient nuclei are not different from the population of other planetary nebulae in the Galaxy. In particular the proportion of the H-deficient stars among young nebulae is the same as in the whole population. We have made a detailed comparison of the observed parameters with theoretical modelling of the late He-shell flash (born again AGB) scenario. Our finding is that the [WC] nuclei are not formed in a late He-shell flash. This scenario can, however, give origin to some PG1159 objects. There are five objects known which have presumably suffered from a late He-shell flash. The observed parameters of their nebulae imply that these stars will not become typical [WC] objects. Thus most of hydrogen-deficient central stars (at least [WC]) evolve directly from the AGB as do the other planetary nebula nuclei. We discuss implications of this result.