Contact and near-contact binary systems - IV. RT Scl and AQ Tuc

Reticon spectroscopic observations are presented for the short-period eclipsing binary systems RT Scl and AQ Tuc. Analysis of these data together with new analyses of previously published light curves show that RT Scl is composed of a main-sequence primary component, virtually in contact with its Roche lobe, and a detached secondary component which is ∼40 per cent larger than expected for its mass. The observed decrease in orbital period and the asymmetry in the light curve may be understood in terms of mass transfer from the primary component at a rate of $$10^{-8}M_\odot \enspace \text {yr}^{-1}$$ via a stream from the inner Lagrangian point which hits directly the surface of the oversized secondary. RT Scl is therefore a good candidate for the broken-contact phase of the thermal relaxation oscillation model for contact binaries. Alternatively, it is close to the same evolutionary state as FT Lupi of evolution into contact for the first time. The system AQ Tuc, which has the same total mass and specific orbital angular momentum as RT Scl, is confirmed to be an A-type WUMa contact system. There is some disagreement between the spectroscopic and photometric determinations of the mass ratio which may result from the small (∼0.02 mag) asymmetries in the light curves and/or from the differences in relative strengths of the spectra of the two components between first quadrature and second quadrature. The best solutions of the light curves are obtained with a convective value ($$\beta=0.03$$) for the gravity-darkening exponent. These findings support the conclusions reached by Eaton that convective (or turbulent) atmospheres extend to earlier spectral types amongst contact systems than amongst single stars.