The evolutionary state of contact and near-contact binary stars

A compilation is presented of the masses, radii and luminosities of the components of 31 F–K type binary systems which have been found to be in contact or near-contact states. Comparisons of these data with those expected for single stars demonstrate that (i) the primary components of the shallow-contact W-type WUMa systems are unevolved main-sequence stars whilst those of the deeper-contact A-type systems are near to the terminal-age main sequence, (ii) the secondary components of the W-type systems have radii of the order of 1.5 times larger than expected for their ZAMS masses whilst those of the A-type systems have radii of the order of three times larger, (iii) the nine systems in the sample which do not display EW-type light curves can be divided into three groups: (a) three marginal-contact systems where both components just fill their respective Roche lobes and display evidence in their light curves of energy exchange between the components, (b) five semi-detached systems with no complications in their light curves, and (c) one detached system which has its primary component close to its Roche lobe. Consideration of these factors together with the specific orbital angular momenta of these systems leads to the suggestions that there are two types of evolution into contact: (i) from detached systems (e.g. RT Scl) via marginal-contact systems (e.g. FT Lup) to the shallow-contact W-type systems, and (ii) from detached systems via case-A mass transfer to semi-detached systems [e.g. CX Aqr, YY Cet, RS Ind, FO Vir, (EE Aqr?)], to marginal-contact systems (e.g. RV Crv, CX Vir) to the deeper-contact A-type systems. Empirical evidence is also presented that the luminosity transfer between the components of W-type systems amounts to changes in log T_eff of −0.05/+0.09 for the primary and secondary components respectively which in turn leads to changes in $$\text {log}\enspace L/L_\odot$$ of −0.20/+0.36. These findings are in good agreement with theoretical expectations.