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 hydrogen, which was recently discovered in the Hamburg Quasar Survey. We have derived temperatures and element abundances from optical and UV spectra by comparison with synthetic line profiles. In addition UV (HST, IUE) and optical spectra of the well-known white dwarf GD 40 with very similar atmospheric parameters were analysed and element abundances determined. For both stars we 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 overabundant 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. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract No. NAS5-26555. Based on observations with the International Ultraviolett Explorer satellite (IUE) collected at Villafranca, Spain. Based on observations collected at the German-Spanish Astronomical Center (DSAZ), Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy.