Chemical abundances of hot post-AGB stars

Context. Hot post-AGB stars are luminous objects of low- and intermediate mass (0.8-8 M-circle dot) in the final stage of evolution, moving between the asymptotic giant branch (AGB) stars and planetary nebulae. The absorption lines observed in their spectra are typical of early-type stars and their abundance pattern may be associated with the occurence of one or more mixing processes during the previous AGB phase. Aims. To better constrain their observed chemical pattern and evolutionary status, we determined chemical abundances for a sample of hot post-AGB stars selected according to spectroscopic criteria. Methods. The observational data are high-resolution spectra obtained with the FEROS spectrograph. The stellar parameters and chemical composition were obtained from fully consistent non-LTE synthesis. Results. The general abundance pattern reveals relevant nitrogen enrichment, slight depletion in carbon and sulfur and mild excess in helium for most of the objects. One notable exception is LSE 148, with Z = 0.001, which is likely to be a metal-poor object at high galactic latitude. The atmospheric parameters and chemical abundances obtained are discussed in the context of evolutionary models. Mixing processes like the second/third dredged-up and "hot bottom-burning" are invoked to explain the obtained results.