  Due to thermal pulses, asymptotic giant branch (AGB) stars experience periods
of convective mixing that provide ideal conditions for slow neutron-capture
nucleosynthesis. These processes are affected by large uncertainties and are
still not fully understood. By the lucky coincidence that about a quarter of
all post-AGB stars turn hydrogen-deficient in a final flash of the
helium-burning shell, they display nuclear processed material at the surface
providing an unique insight to nucleosynthesis and mixing. We present results
of non-local thermodynamic equilibrium spectral analyses of the extremely hot,
hydrogen-deficient, PG 1159-type central stars of the Skull Nebula NGC 246 and
the "Galactic Soccerballs" Abell 43 and NGC 7094.
