2019RAA....19...14K

We present photometric observations in Sloan filters g', i' of the eclipsing W UMa stars USNO-A2.0 1350-17365531, V471 Cas, V479 Lac and V560 Lac. The sinusoidal-like O - C diagram of V471 Cas indicates the presence of a third body with mass 0.12 M_☉ (a red dwarf) at distance 897 R_☉. The O - C diagram of V479 Lac reveals a period decrease of dP/dt = -1.69 x 10^–6 d yr^–1. The results of the light curve solutions are: (i) the targets are overcontact binaries with small fill-out factors; (ii) their components are F-K stars, comparable in size, whose temperature differences are below 80 K; (iii) all targets undergo partial eclipses and to limit the possible mass ratios we carried out two-step q-search analysis. The target global parameters (luminosities, radii, masses) were obtained on the basis of their Gaia distances and the results of our light curve solutions. The obtained total mass of V560 Lac turns out to be smaller than the lower mass limit for presently known W UMa binaries of 1.0 - 1.2 M_☉, i.e. this target is a peculiar overcontact system.