Abundances of [WC] central stars and their planetary nebulae

We review elemental abundances derived for planetary nebula (PN) WC central stars and for their nebulae. Uncertainties in the abundances of [WC] stars are still too large to enable an abundance sequence to be constructed. In particular it is not clear why the hotter [WCE] stars have C and O abundanc es which are systematically lower than those of their supposed precursors, the [WCL] stars. This abundance difference could be real or it may be due t o unaccounted-for systematic effects in the analyses. Hydrogen might not be present in [WC] star winds as originally suggested, since broad pedestals observed at the base of nebular lines can plausibly be attributed to high v elocity nebular components. It is recommended that stellar abundance analys es should be carried out with non-LTE model codes, although recombination l ine analyses can provide useful insights. In particular, C II dielectronic recombination lines provide a unique means to determine electron temperatur es in cool [WC] star winds. We then compare the abundances found for PNe wh ich have [WC] central stars with those that do not. Numerous abundance anal yses of PNe have been published, but comparisons based on non-uniform sampl es and methods are likely to lack reliability. Nebular C/H ratios, which mi ght be expected to distinguish between PNe around H-poor and H-rich stars, are rather similar for the two groups, with only a small tendency towards l arger values for nebulae around H-deficient stars. Nebular abundances shoul d be obtained with photoionization models using the best-fitting non-LTE mo del atmosphere for the central star as the input. Heavy-metal line blanketi ng still needs to be taken into consideration when modeling the central sta r, as its omission can significantly affect the ionizing fluxes as well as the abundance determinations. We discuss the discrepancies between nebular abundances derived from collisionally excited lines and those derived from optical recombination lines, a phenomenon that may have links with the pres ence of H-deficient central stars.