  We report the first discovery of argon in hot evolved stars and white dwarfs.
We have identified the ArVII 1063.55A line in some of the hottest known
(Teff=95000-110000 K) central stars of planetary nebulae and (pre-) white
dwarfs of various spectral type. We determine the argon abundance and compare
it to theoretical predictions from stellar evolution theory as well as from
diffusion calculations. We analyze high-resolution spectra taken with the Far
Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model
atmospheres and perform line-formation calculations to compute synthetic argon
line profiles. We find a solar argon abundance in the H-rich central star
NGC1360 and in the H-deficient PG1159 star PG1424+535. This confirms stellar
evolution modeling that predicts that the argon abundance remains almost
unaffected by nucleosynthesis. For the DAO-type central star NGC7293 and the
hot DA white dwarfs PG0948+534 and REJ1738+669 we find argon abundances that
are up to three orders of magnitude smaller than predictions of calculations
assuming equilibrium of radiative levitation and gravitational settling. For
the hot DO white dwarf PG1034+001 the theoretical overprediction amounts to one
dex. Our results confirm predictions from stellar nucleosynthesis calculations
for the argon abundance in AGB stars. The argon abundance found in hot white
dwarfs, however, is another drastic example that the current state of
equilibrium theory for trace elements fails to explain the observations
quantitatively.
