ROSAT EUV AND SOFT-X-RAY STUDIES OF ATMOSPHERIC COMPOSITION AND STRUCTURE IN G191-B2B

Previous studies of the hot DA white dwarf G191-B2B have been unable to determine whether the observed soft X-ray and EUV opacity arises from a stratified hydrogen and helium atmosphere or from the presence of trace metals in the photosphere. New EUV and soft X-ray photometry of this star, made with the ROSAT observatory, when analysed in conjunction with the earlier data, shows that the stratified models cannot account for the observed fluxes. Consequently, we conclude that trace metals must be a substantial source of opacity in the photosphere of G191-B2B.