  Two UV brights stars in globular clusters, ROA 5701 (omega Cen) and Barnard
29 (M 13) are analysed from high-resolution UV and optical spectra. The main
aim is the measurement of iron abundances from UV spectra obtained with the
HST-GHRS. In addition atmospheric parameters and abundances for He, C, N, O,
and Si are derived from optical spectra (ESO CASPEC) for ROA 5701 or taken from
literature for Barnard 29. Both stars are found to be post-asymptotic giant
branch stars. Surprisingly, their iron abundances lie significantly below the
cluster abundance in both cases. Barnard 29 lies 0.5 dex below the iron
abundance derived for giant stars in M 13 and the iron abundance of ROA 5701 is
the lowest of any star in omega Cen analysed so far. Barnard 29 shows the same
abundance pattern as the red giant stars in M 13, except for its stronger iron
deficiency. The iron depletion could be explained by gas-dust separation in the
AGB progenitor's atmosphere, if iron condensed into dust grains which were then
removed from the atmosphere by a radiatively driven wind. The interpretation of
the abundance pattern for ROA 5701 is hampered by the star-to-star abundance
variations seen in omega Cen, but its abundance pattern appears to be
consistent with the gas-dust separation scenario.
