Metal-line blanketing and the peculiar H-beta line profile in the DAO star Feige 55

A detailed analysis of the optical spectrum of the DAO white dwarf Feige 55 is presented. We show that the atmospheric parameters obtained from fits to the H-beta line and to higher Balmer lines, respectively, are discrepant. This problem is akin to that reported in previous analyses of hot DAO central stars of planetary nebulae. Model atmosphere calculations are used to demonstrate that the discrepancy is likely to be due to the presence of small quantities of heavy elements in the atmosphere of Feige 55. We then show that line blanketing by these elements indeed produces enough cooling of the upper layers to change significantly the predicted line profiles. New models incorporating this opacity source provide excellent agreement with the observed profiles. We discuss several implications of our result on the current analysis of X-ray and EUV fluxes of hot DA and DAO stars.