  There is a striking paucity of hydrogen-rich (DA) white dwarfs (WDs) relative
to their hydrogen-deficient (non-DA) counterparts at the very hot end of the WD
cooling sequence. The three hottest known DAs (surface gravity log g $\geq$
7.0) have effective temperatures around Teff = 140,000 K, followed by only five
objects in the range 104,000 - 120,000 K. They are by far outnumbered by forty
non-DAs with Teff = 100,000 - 250,000 K, giving a DA/non-DA ratio of 0.2. In
contrast, this ratio is the inverse of that for the cooler WDs. One reason for
this discrepancy could be uncertainties in the temperature determination of hot
DAs using Balmer-line spectroscopy. Recent investigations involving
metal-ionization balances in ultraviolet (UV) spectra indeed showed that the
temperatures of some DAs were underestimated, but the paucity of extremely hot
DAs prevailed. Here we present the results of a UV spectral analysis of one of
the three hottest DAs, PG0948+534. We find that its temperature was strongly
overestimated by recent Balmer line analyses. We correct it downward to 105,000
$\pm$ 5000 K, aggravating the hot-DA paucity. The Balmer-line problem
encountered previously is not resolved by our non-LTE line-blanketed model
atmospheres. We speculate that it might be related to the possible presence of
a magnetosphere. This is supported by the V-band variability that shows a
period of P=3.45 d (amplitude 0.19 mag), which we interpret as the star's
rotation period. The metal abundances in PG0948+534 are affected by atomic
diffusion and we conclude that the onset of diffusion in hot DAs occurs when
they cool below Teff about 105,000 K. We discuss the possibility that the
paucity of very hot DAs is a consequence of their fast evolutionary rate.
