The Binary Double-Degenerate Nature of the Bright DAO White Dwarf Feige 55

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Abstract
Recent IUE and optical spectra of the bright DAO white dwarf Feige 55 reveal large Doppler shifts due to orbital motion. These Doppler shifts have a period of 1.49303 ± 0.00011 days with a semi-amplitude of 77.4 ± 7.0 km s-1. There is no strong photometric signature of any companion; J- and K-band photometry place an upper limit of 0.1 M☉ on the mass of any main sequence secondary. From the mass function, we derive a lower limit of 0.25 M☉ for the mass of the companion. We conclude that the unseen component is a cooler, less luminous, degenerate star. The binary nature of Feige 55 strongly suggests that the system is the product of common envelope evolution and that the lowest mass DAO stars studied by Bergeron et al. are also double-degenerates.