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Atmospheric parameters for hot DB (helium atmosphere) white dwarfs neareffective temperatures of 25000K are extremely difficult to determine fromoptical spectroscopy. This is particularly unfortunate, because this is therange of variable DBV or V777 Her stars. Accurate atmospheric parameters areneeded to help or confirm the asteroseismic analysis of these objects. Anotherimportant aspect is the new class of white dwarfs - the hot DQ - detected byDufour et al. (2007), with spectra dominated by carbon lines. The analysisshows that their atmospheres are pure carbon. The origin of these stars is notyet understood, but they may have an evolutionary link with the hotter DBs asstudied here. Our aim is to determine accurate atmospheric parameters andelement abundances and study the implications for the evolution white dwarfs ofspectral classes DB and hot DQ. High resolution UV spectra of five DBs arestudied with model atmospheres. We determine stellar parameters and abundancesor upper limits of C and Si. These objects are compared with cooler DBs below20000K. We find photospheric C and no other heavy elements - with extremelyhigh limits on the C/Si ratio - in two of the five hot DBs. We compare variousexplanations for this unusual composition, which have been proposed in theliterature: accretion of interstellar or circumstellar matter, radiativelevitation, carbon dredge-up from deeper interior below the helium layer, and aresidual stellar wind. None of these explanations is completely satisfactory,and the problem of the origin of the hot DQ remains an open question.

Atmospheric parameters and carbon abundance for hot DB white dwarfs