Long-Period Eclipsing Binaries: VV Cephei and Epsilon Aurigae

Abstract

(a) Apparent orbit vs annual parallax. A determination of the astrometric orbit of a spectroscopic binary is of interest for its own sake but also because it may permit the determination of a precise (absolute) parallax if the linear dimension of the orbit is sufficiently large. This situation turns out to be of particular interest for the two eclipsing spectroscopic binaries of known longest periods, 20.4 and 27.08 yr, for the naked-eye stars VV Cephei and Epsilon Aurigae respectively. In each of these systems the brighter component is an extreme supergiant. The primary of VV Cephei may well be the largest known red supergiant star, so extended that with the Sun at its center the orbit of Mars would easily be contained inside it. The primary of Epsilon Aurigae has a diameter exceeding that of the Earth’s orbit, and may well be the F star of highest known luminosity; its massive invisible companion remains a mysterious object.