A study of four contact binary systems using the DDO photometric system

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Abstract
Complete light curves and colour index curves on the DDO photometric system are presented for the W-type contact binary systems W UMa, XY Leo, TX Cnc and AH Vir. The data have been analysed using Rucinski's light curve synthesis code under a variety of constraints. It is found that the gravity darkening exponent must be small (β ≃ 0.00) for these late-type stars (F8–K2). It then follows that the degree of contact between the components is of the order of 25 per cent, rather than the 15 per cent derived if Lucy's value of β = 0.08 is adopted, and that there is some indication that the discrepancy between photometric and spectroscopic mass ratios is reduced when β = 0.00 is used. The significant asymmetries in the light curves of W UMa and AH Vir observed at this epoch may be readily understood in terms of starspots of small extent (≲ 10 per cent of the visible surface of one component).

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