2000A&A...354..135V

We present in this paper a homogeneous photospheric abundance study, on the basis of the analysis of high resolution optical spectra, of six post-AGB objects displaying a 21µm circumstellar dust feature in their IR spectrum. The F-G spectral type of the 21µm stars make that a large range of elements including a wide variety of s-process elements, can be studied by their atomic lines. The high C/O-ratios together with the large overabundance of s-process elements prove that the objects are descendants of field carbon stars. We discuss in detail the s-process abundance distribution displayed by these 21µm stars and conclude that the 3rd dredge-up efficiency is closely related to the strength of the integrated neutron irradiation. The expected anti-correlation of the neutron irradiation with metallicity, on the other hand, contains a large intrinsic scatter. Finally we compare our results with other intrinsic and extrinsic s-process enriched objects and conclude that the post-AGB stars offer very useful complementary data to constrain the evolutionary models of AGB nucleosynthesis and dredge-up processes.