2.6 W UMa stars

W UMa stars are eclipsing binaries with periods between about $5$ and $20 h$ showing continuous light variations (Selam, 2004 ). They consist of two solar-type components surrounded by a common envelope. A large-scale energy transfer from the larger, more massive component to the smaller, less massive one results in almost equalising surface temperatures over the entire system. The components of such a contact binary rotate very rapidly in spite of their old ages ( $v sin i ~ 100- 200 km s-1$ ) as a result of spin-orbit synchronisation due to strong tidal interactions between the stars. W UMa stars are very common, comprising some 95% of eclipsing binary variables in the solar neighbourhood, or one star in every 1000 - 2000 in the same spectral range (Eggen, 1967 ).

Observations reveal that there are two subclasses of W UMa stars: A-type and W-type systems. The former have longer periods, are hotter, have larger total mass, and a smaller mass-ratio and are in better contact. The effective temperatures of the primary and secondary stars in W Uma-type variables are not exactly the same. In W-type the temperature difference is larger and the secondary appears hotter, while in A-type the primary is hotter or almost the same temperature as the secondary. Mullan (1975 ) suggested that the primary stars in W-type systems appear cooler relative to the secondary due to cool magnetic starspots on their surfaces. Following this idea, photometric peculiarities of such systems have been often interpreted with the help of either cool or hot spots on the binary components. A study of the contact binaries with Doppler imaging technique (see Section 4.2) reveals that both components can be covered by cool starspots, with a tendency for the primary to be more active than the secondary (Maceroni et al., 1994 ; Hendry and Mochnacki, 2000 ; Barnes et al., 2004 ).