Photometry of two DQ white dwarfs. Search for spots

Swan bands in cool DQ white dwarfs cannot be adequately fitted with models that otherwisesuccesfully reproduce spectral features of the molecule CH in these stars. Initial modelling showed that a two-component atmospherein the style of a spot might be able to solve the problem.Methods.We photometrically observed the two cool DQ white dwarfs GJ1117 and EGGR78 to search for variability caused by stellarspots.Results.We have not found any such variability, but we estimate the effects of hypothetical spots on lightcurves. We also estimatedetection probabilities for spots in different configurations. Alternative explanations of the prob lem are needed and briefly discussed.Key words. white dwarfs – stars: atmosphere – stars: spots