  The relatively high abundance of carbon in the hot DO white dwarf RE0503-289
indicates that it is a descendant of a PG1159 star. This is corroborated by the
recent detection of the extremely high abundances of trans-Fe elements which
stem from s-process nucleosynthesis in the precursor AGB star, dredged up by a
late He-shell flash and possibly amplified by radiative levitation. On the
other hand, the hottest known DO white dwarf, KPD0005+5106, cannot have evolved
from a PG1159 star but represents a distinct He-rich evolutionary sequence that
possibly originates from a binary white dwarf merger.
