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For the first time, we have identified photospheric emission lines in thefar-UV spectrum of a white dwarf. They were discovered in the Far UltravioletSpectroscopic Explorer spectrum of the hot (Teff~200,000 K) DO white dwarfKPD0005+5106 and they stem from extremely highly ionized calcium (CaX 1137,1159 Ang). Their photospheric origin is confirmed by non-LTE line-formationcalculations. This is the highest ionisation stage of any element ever observedin a stellar photosphere. Calcium has never been detected before in any hotwhite dwarf or central star of planetary nebula. The calcium abundancedetermination for KPD0005+5106 (1-10 times solar) is difficult, because theline strengths are rather sensitive to current uncertainties in the knowledgeof effective temperature and surface gravity. We discuss the possibility thatthe calcium abundance is much lower than expected from diffusion/levitationequilibrium theory. The same emission lines are exhibited by the [WCE]-typecentral star NGC2371. Another CaX line pair (1461, 1504 Ang) is probablypresent in a Hubble Space Telescope spectrum of the PG1159-type central starNGC246.

Discovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD0005+5106)