  We construct a data base of 125 post-AGB objects (including R CrB and extreme
helium stars) with published photospheric parameters (effective temperature and
gravity) and chemical composition. We estimate the masses of the post-AGB stars
by comparing their position in the (log Teff, log g) plane with theoretical
evolutionary tracks of different masses. We construct various diagrams, with
the aim of finding clues to AGB nucleosynthesis. This is the first time that a
large sample of post-AGB stars has been used in a systematic way for such a
purpose and we argue that, in several respects, post-AGB stars should be more
powerful than planetary nebulae to test AGB nucleosynthesis. Our main findings
are that: the vast majority of objects which do not show evidence of N
production from primary C have a low stellar mass (M* < 0.56 Msun); there is no
evidence that objects which did not experience 3rd dredge-up have a different
stellar mass distribution than objects that did; there is clear evidence that
3rd dredge-up is more efficient at low metallicity. The sample of known
post-AGB stars is likely to increase significantly in the near future thanks to
the ASTRO-F and follow-up observations, making these objects even more
promising as testbeds for AGB nucleosynthesis.
