Abstract

The spectroscopy of rapidly rotating binary stars occasionally encounters systems where - paradoxically - the primary is effectively hidden in the light of its secondary. Here, we report on a bright textbook example of this kind with the nearby A-type star alpha Lib A. Although discovered as a spectroscopic binary already in the year 1904, a radial velocity curve has never been published up to a point where the existence of the secondary became even suspicious. However, in this work, we demonstrate that alpha Lib A is indeed a double-lined spectroscopic binary with a rapidly rotating - and hence almost invisible - A-type primary, accompanied and predominated by a more slowly rotating late-A or early F-type secondary in an eccentric P = 70.34 d orbit. On account of the shallow absorption lines of the primary, uncertainties remain with its semi-amplitude and hence the exact mass ratio. Yet, with a maximal projected separation that should lie in the range 20 to 25 mas, follow-up high-angular resolution observations might soon establish alpha Lib Aa-Ab as a visual binary, with a measure for the orbital inclination and precisely determined stellar masses.