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For the first time, we have identified NeVIII absorption lines in far-UVspectra of the hottest known (Teff>150,000 K) hydrogen-deficient (pre-) whitedwarfs of spectral type PG1159. They are of photospheric origin and can bematched by synthetic non-LTE line profiles. We also show that a number of UVand optical emission lines in these stars can be explained as beingphotospheric NeVIII features and not, as hitherto suspected, as ultrahighionised OVIII lines created along shock-zones in the stellar wind.Consequently, we argue that the long-standing identification of the sameemission lines in hot [WR]-type central stars as being due to ultrahigh-ionisedspecies (OVII-VIII, CV-VI) must be revised. These lines can be entirelyattributed to thermally excited species (NeVII-VIII, NV, OVI). PhotosphericNeVIII lines are also identified in the hottest known He-rich white dwarfKPD0005+5106 some of which were also attributed to OVIII previously. This is asurprise because it must be concluded that KPD0005+5106 is much hotter(Teff=200,000 K) than hitherto assumed (Teff=120,000 K). This is confirmed by are-assessment of the HeII line spectrum. We speculate that the temperature ishigh enough to explain the mysterious, hard X-ray emission (1 keV) as being ofphotospheric origin.

Identification of NeVIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars