Discovery of 12 short-period eclipsing binaries in the old open cluster Berkeley 39

We report the results of a CCD search for short-period variables in the field of the old open cluster Berkeley 39. Twelve variables were discovered among ≃ 2000 stars with I>20.0. We argue that all or almost all discovered variables are members of Be 39. Eleven variables are contact binaries (W UMa-type systems) with periods ranging from 0.225 to 0.546 d. The twelfth variable is probably an eclipsing binary with a period of the order of 1 d. Four variables are located among the blue stragglers on the cluster colour-magnitude diagram. The distribution of binaries along this diagram (a 5-mag spread) favours the theory that the Berkeley 39 W UMa-type systems have been formed mainly through the mechanism of magnetic braking