  The observational data guiding the theoretical chemical evolutionary models
of AGB stars come mainly from the analysis of intrinsic and extrinsic s-process
enriched objects. The first are the stars of the M-MS-S-SC-C star sequence
which is thought to reflect, at least partly, the evolution on the AGB of a
single star towards an increasing C/O ratio. The second are all binaries with
excess abundances acquired by mass-transfer from the companion which is now a
white dwarf.
  In this contribution we present a homogeneous abundance analysis of 21micron
objects, which are carbon-rich probably single post-AGB stars. With their F to
G spectral types a wide variety of chemical species can be studied
quantitatively by using photospheric atomic lines. These objects are among the
most s-process enriched objects known so far. We focus on the s-process
distribution and the related neutron irradiation and compare the results with
the enrichments observed in both intrinsic and extrinsic objects.
