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In an effort to extend the still limited sample of s-process enrichedpost-AGB stars, high-resolution, high signal-to-noise VLT+UVES spectra of theoptical counterparts of the infrared sources IRAS06530-0213 and IRAS08143-4406were analysed. The objects are moderately metal deficient by [Fe/H]=-0.5 and-0.4 respectively, carbon-rich and, above all, heavily s-process enhanced witha [ls/Fe] of 1.8 and 1.5 respectively. Especially the spectrum ofIRAS06530-0213 is dominated by transitions of s-process species, and thereforeresembling the spectrum of IRAS05341+0852, the most s-process enriched objectknown so far. The two objects are chemically very similar to the 21micronobjects discussed in Van Winckel & Reyniers (2000). A homogeneous comparisonwith the results of these objects reveals that the relation between the thirddredge-up efficiency and the neutron nucleosynthesis efficiency found for the21micron objects, is further strengthened. On the other hand, a detailedcomparison with the predictions of the latest AGB models indicates that theobserved spread in nucleosynthesis efficiency is certainly intrinsic, andproves that different C-13 pockets are needed for stars with comparable massand metallicity to explain their abundances.Comment: 14 pages, 10 figures, accepted for publication in A&A; Table 4 is  available at ftp://ftp.ster.kuleuven.ac.be/dist/maarten/filescds/ pending  upload to CD

A study of the s-process in the carbon-rich post-AGB stars IRAS 06530–0213 and IRAS 08143–4406 on the basis of VLT-UVES spectra