About 25 % of all post-AGB stars are hydrogen-deficient , e.g .
the PG 1159 stars with a typical abundance pattern He : C : O = 33:50:17 ( by mass ) .
Only four of about 40 known PG 1159 stars exhibit H in their spectra .
The exciting star of the planetary nebula A 43 is one of these so-called hybrid PG 1159 stars .
We present preliminary results of an on-going spectral analysis by means of NLTE model-atmosphere techniques based on UV spectra obtained with FUSE , HST / GHRS , and IUE as well as on optical observations . Keywords . ISM : planetary nebulae : individual : A 43 - Stars : abundances - Stars : atmospheres - Stars : evolution - Stars : individual : WD 1721+106 - Stars : AGB and post-AGB