[en] 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 = approximately 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 (< approximately 10 per cent of the visible surface of one component). (author)