  Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near
effective temperatures of 25000K are extremely difficult to determine from
optical spectroscopy. This is particularly unfortunate, because this is the
range of variable DBV or V777 Her stars. Accurate atmospheric parameters are
needed to help or confirm the asteroseismic analysis of these objects. Another
important aspect is the new class of white dwarfs - the hot DQ - detected by
Dufour et al. (2007), with spectra dominated by carbon lines. The analysis
shows that their atmospheres are pure carbon. The origin of these stars is not
yet understood, but they may have an evolutionary link with the hotter DBs as
studied here. Our aim is to determine accurate atmospheric parameters and
element abundances and study the implications for the evolution white dwarfs of
spectral classes DB and hot DQ. High resolution UV spectra of five DBs are
studied with model atmospheres. We determine stellar parameters and abundances
or upper limits of C and Si. These objects are compared with cooler DBs below
20000K. We find photospheric C and no other heavy elements - with extremely
high limits on the C/Si ratio - in two of the five hot DBs. We compare various
explanations for this unusual composition, which have been proposed in the
literature: accretion of interstellar or circumstellar matter, radiative
levitation, carbon dredge-up from deeper interior below the helium layer, and a
residual stellar wind. None of these explanations is completely satisfactory,
and the problem of the origin of the hot DQ remains an open question.
