Spectroscopic Analysis of the DAB White Dwarf PG 1115 + 166 : An Unresolved DA + DB Degenerate Binary

A spectroscopic analysis of the DAB white dwarf PG 1115+166 is presented. The observed hydrogen and helium line profiles are shown to be incompatible with model spectra calculated under the assumption of homogeneous or stratified chemical compositions. In contrast, an excellent fit to the optical spectrum of PG 1115+166 can be achieved if the object is interpreted as an unresolved double degenerate composed of a hydrogen-line DA star and a helium-line DB star. The atmospheric parameters obtained from the best fit are Teff = 22, 090 K and log g = 8.12 for the DA star, Teff = 16, 210 K and log g = 8.19 for the DB star. This binary interpretation is consistent with the results recently reported by Burleigh et al. that PG 1115+166 also exhibits radial velocity variations. The implications of this discovery with respect to the DAB spectral class are discussed. Subject headings: stars: binaries, stars: individual (PG 1115+166), white dwarfs