  Among the spectroscopically identified white dwarfs, a fraction smaller than
2% have spectra dominated by carbon lines, mainly molecular C2, but also in a
smaller group by CI and CII lines. These are together called DQ white dwarfs.
We want to derive atmospheric parameters Teff,log g, and carbon abundances for
a large sample of these stars and discuss implications for their spectral
evolution. Sloan Digital Sky Survey spectra and ugriz photometry were used,
together with GAIA Data Release 2 parallaxes and G band photometry. These were
fitted to synthetic spectra and theoretical photometry derived from model
atmospheres. We found that the DQs hotter than Teff ~10000 K have masses ~0.4
Msun larger than the cooler ones, which have masses typical for the majority of
white dwarfs, ~0.6 Msun. A significant fraction of the hotter objects with Teff
> 14500 K have atmospheres dominated by carbon.
