  The hydrogen-deficiency in extremely hot post-AGB stars of spectral class
PG1159 is probably caused by a (very) late helium-shell flash or a AGB final
thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden
intershell region. Thus, the photospheric elemental abundances of these stars
allow to draw conclusions about details of nuclear burning and mixing processes
in the precursor AGB stars. We compare predicted elemental abundances to those
determined by quantitative spectral analyses performed with advanced non-LTE
model atmospheres. A good qualitative and quantitative agreement is found for
many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S,
Fe) point at shortcomings in stellar evolution models for AGB stars. PG1159
stars appear to be the direct progeny of [WC] stars.
