  The determination of effective temperatures of very hot central stars
(Teff>70000K) by model atmosphere analyses of optical H and He line profiles is
afflicted with considerable uncertainty, primarily due to the lack of neutral
helium lines. Ionization balances of metals, accessible only with UV lines,
allow more precise temperature estimates. The potential of iron lines is
pointed out. At the same time iron and other metal abundances, hardly
investigated until today, may be derived from UV spectra. We describe recent
HST spectroscopy performed for this purpose.
  A search for iron lines in FUV spectra of the hottest H-deficient central
stars (PG1159-type, Teff>100000K) taken with FUSE was unsuccessful. The derived
deficiency is interpreted in terms of iron depletion due to n-capture
nucleosynthesis in intershell matter, which is now exposed at the stellar
surface as a consequence of a late He shell flash.
