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 abundances which are systematically lower than those of their supposed precursors, the [WCL] stars. This abundance difference could be real or it may be due to 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 velocity nebular components. It is recommended that stellar abundance analyses should be carried out with non-LTE model codes, although recombination line analyses can provide useful insights. In particular, C II dielectronic recombination lines provide a unique means to determine electron temperatures in cool [WC] star winds. We then compare the abundances found for PNe which have [WC] central stars with those that do not. Numerous abundance analyses of PNe have been published, but comparisons based on non-uniform samples and methods are likely to lack reliability. Nebular C/H ratios, which might be expected to distinguish between PNe around H-poor and H-rich stars, are rather similar tor the two groups, with only a small tendency towards larger values for nebulae around H-deficient stars. Nebular abundances should be obtained with photoionization models using the best-fitting non-LTE model atmosphere for the central star as the input. Heavy-metal line blanketing still needs to be taken into consideration when modeling the central star, 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 presence of H-deficient central stars.