  We present an evolutionary sequence of a low mass star from the Asymptotic
Giant Branch (AGB) through its post-AGB stage, during which its surface
chemical composition changes from hydrogen-rich to strongly hydrogen-deficient
as consequence of a very late thermal pulse, following the so-called born-again
scenario. The internal structure and abundance changes during this pulse are
computed with a numerical method which allows the physically consistent
calculation of stellar layers where thermonuclear and mixing time scale are
comparable - a situation which occurs when the helium flash driven convection
zone extends to the hydrogen-rich surface layers during the pulse peak. The
final surface mass fractions are [He/C/O]=[0.38/0.36/0.22], where the high
oxygen abundance is due to diffusive overshoot employed during the AGB
evolution. These models are the first to achieve general agreement with the
surface abundance pattern observed in hydrogen-deficient post-AGB stars - e.g.
the PG 1159 stars or the WR-type central stars of planetary nebulae -,
confirming the born-again scenario with a physically consistent calculation and
supporting the occurrence of convective overshooting in thermally pulsing AGB
stars.
