The orbital period of the pre-cataclysmic binary RE 2013+400 and a study of the atmosphere of the DAO white dwarf primary

• Published in: Monthly notices of the Royal Astronomical Society 1995-02, Vol.272 (3), p.531-543, Article 531
• Main Authors: Barstow, M. A., Burleigh, M. R., Fleming, T. A., Holberg, J. B., Koester, D., Marsh, M. C., Rosen, S. R., Rutten, R. G. M., Sakai, S., Tweedy, R. W., Wegner, G.
• Format: Article
• Language: English
• Subjects: binaries: close; stars: atmospheres; ultraviolet: stars; white dwarfs; X-rays: stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/272.3.531

Several pre-cataclysmic binaries, comprising a hot white dwarf with a red dwarf companion, have been discovered as a result of the optical identification of EUV sources from the ROSAT all-sky survey. The optical spectra have the steep blue continuum and Balmer absorption typical of a hot white dwarf, but there are bright, narrow emission lines of H I (and sometimes He I and Ca II) superimposed. An intense campaign of follow-up observations has been devoted to these binary systems. So far, only RE 2013+400 has exhibited any measurable changes in the radial velocities of the emission components, from which it is possible to estimate that the binary period is 0.71 d. A clear He II 4886-Å absorption feature is detected, which indicates that, like most PCBs with white dwarfs hotter than 40 000 K, the white dwarf is a hydrogen-helium hybrid star (DAO). A combined analysis of the optical, UV and EUV/X-ray data suggests that the atmospheric He abundance is higher in the optical line-forming region of the white dwarf photosphere than in the region where the EUV/X-ray flux is formed. This is an interesting result, in the light of the recent optical study of a sample of DAO white dwarfs by Bergeron et al., if representative of DAO white dwarfs in general.