  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 21mic circumstellar dust feature in their IR spectrum. The
F-G spectral type of the 21mic 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 21mic
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.
