BI Lyncis: A hydrogen-deficient binary consisting of two low-mass giants of spectral types early-B and G

Abstract

BI Lyn has previously been misclassified as an evolved binary system containing either a hot subdwarf or white dwarf and a thick accretion disk. New intermediate dispersion spectra are presented which demonstrate clearly that the hot component is a luminous low-mass helium star and the cool companion is a rapidly rotating G-type giant. Techniques of spectrum synthesis have been used to establish the dimensions of both components. Although the orbital period of the system remains unknown, other phenomena are entirely consistent with these observed dimensions. A ~ 0.34 d periodic photometric variation could be explained by pulsations in the B-type primary and, by analogy with other H-deficient binaries, it is suggested that the variable absorption in Hα may be due to a gas stream flowing through the inner Lagrangian point. Based on observations obtained with the Isaac Newton and William Herschel Telescopes, and on INES data from the IUE satellite.