The determination of effective temperatures of very hot central stars ( T _ { eff } > 70 000 K ) 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 , T _ { eff } > 100 000 K ) 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 .