  PG1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres
mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is
the result of a late helium-shell flash. Observed element abundances enable us
to test stellar evolution models quantitatively with respect to their
nucleosynthesis products formed near the helium-burning shell of the progenitor
asymptotic giant branch stars. Because of the high effective temperatures
(Teff), abundance determinations require ultraviolet spectroscopy and non-local
thermodynamic equilibrium model atmosphere analyses. Up to now, we have
presented results for the prototype of this spectral class and two cooler
members (Teff in the range 85,000-140,000 K). Here we report on the results for
two even hotter stars (PG1520+525 and PG1144+005, both with Teff = 150,000 K)
which are the only two objects in this temperature-gravity region for which
useful far-ultraviolet spectra are available, and revisit the prototype star.
Previous results on the abundances of some species are confirmed, while results
on others (Si, P, S) are revised. In particular, a solar abundance of sulphur
is measured in contrast to earlier claims of a strong S deficiency that
contradicted stellar evolution models. For the first time, we assess the
abundances of Na, Al, and Cl with newly constructed non-LTE model atoms.
Besides the main constituents (He, C, O), we determine the abundances (or upper
limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good
agreement with stellar models is found.
