Three unresolved double degenerate binaries

Abstract

We present an atmospheric analysis of the DA white dwarfs G4-34, GD 402, and GD 387. The energy distributions and hydrogen line profiles of these stars are shown to be inconsistent with those obtained from single or double DA stars. Our result indicates that these stars are probably composite systems consisting of pairs of unresolved DA and DC stars, with the featureless component diluting the hydrogen line strengths of the DA white dwarf. An algorithm is developed to constrain the atmospheric parameters of the components of each system. Some possible implications of the existence of such DA+DC composite systems on the frequency of close, double degenerate binary systems are discussed.