Analysis of UV and optical spectra of the helium-rich white dwarfs HS 2253+8023 and GD 40

HS 2253+8023 is a helium-rich white dwarf with traces of metals and hydroge n, which was recently discovered in the Hamburg Quasar Survey. We have deri ved temperatures and element abundances from optical and UV spectra by comp arison with synthetic line profiles. In addition UV (HST, IUE) and optical spectra of the well-known white dwarf GD 40 with very similar atmospheric p arameters were analysed and element abundances determined. For both stars w e derive an effective temperature of about 15000 K. Within the framework of the diffusion-accretion theory, metal-to-metal ratios are consistent with accretion in solar element proportions, except for calcium which might be o verabundant in GD 40. However the accretion of hydrogen must be reduced by a factor of 10(6) with respect to the predicted metal-to-hydrogen ratio in order to explain the observed abundances.