Abstract
In a programme to understand the evolution of contact binaries, we have re-observed EQ Cep, ER Cep and V371 Cep, which are members of the old open cluster NGC 188, photometrically. Combining our photometric solutions with some known parameters of the cluster, we derived the mass and the radius for each component. Meanwhile, we investigated the period changes of the three systems, finding a long-term increase of period in every system and discovering two cyclic variations in ER Cep besides the long-term changes (T(3) = 5.40 +/- 0.01 yr, A(3) = 0.0135 +/- 0.0016 d; T(4) = 17.6 +/- 0.1 yr). From the derived physical parameters and analysis of the orbital period, we conclude that (i) both components of V371 Cep have very possibly evolved off the main sequence, as a result of the derived accelerations of gravity log g(1) = 3.93 and log g(2) = 3.98, and (ii) ER Cep is at least a triple, or even quadruple, system, something suggested by the overmassive derived total mass (similar to 3.51 M(circle dot)) and the 5.4-yr-cycle periodic oscillation.
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