  Hydrogen-deficient central stars of planetary nebula such as Wolf-Rayet and
PG1159 central stars and some weak emission line stars are primarily composed
of helium and carbon. This abundance is well explained by a scenario where a
single post-AGB star experiences a last helium shell flash which ingests and
burns, or simply dilutes, the remaining hydrogen atmosphere. But despite its
success in matching the photospheric abundances of these stars, this scenario
is faced with several observational challenges. A binary scenario is proposed
here as a more natural way to face some of the most stringent observational
constraints. In this scenario the H-rich primary in a close binary formed
during a common envelope on the AGB, suffers a last helium shell flash, which
results in a H-deficient primary with some of the characteristics needed to
match the observations.
