2015MNRAS.448.2890D

We carried out photometric and low-resolution spectral observations of six eclipsing ultrashort-period binaries with main-sequence (MS)components. The light-curve solutions of the Rozhen observations show that all targets are overcontact systems. We found a well-defined empirical relation between period and semi-major axis for the short-period binaries and used it for estimation of the global parameters of the targets. Our results revealed that NSVS 925605 is quite an interesting target: (i) it is one of a few contact binaries with M components; (ii) it exhibits high activity (emission in the Hα line, X-ray emission, large cool spots, non-Planck energy distribution); (iii) its components differ in temperature by 700 K. All the appearances of high magnetic activity and the huge fill-out factor (0.7) of NSVS 925605 may be a precursor of the predicted merging of close magnetic binaries. Another unusual binary is NSVS 2700153, which reveals considerable long-term variability.