We carried out photometric and low-resolution spectral observations of six eclipsing ultrashort-period binaries with MS components .
The light curve solutions of the Rozhen observations show that all targets are overcontact systems .
We found well-defined empirical relation “ period – 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 interesting target : ( a ) it is one of a few contact binaries with M components ; ( b ) it exhibits high activity ( emission in H \alpha line , X-ray emission , large cool spots , non-Planck energy distribution ) ; ( c ) its components differ in temperature by 700 K. All appearances of high magnetic activity and huge fillout factor ( 0.7 ) of NSVS 925605 might be assumed as a precursor of the predicted merging of close magnetic binaries .
Another unusual binary is NSVS 2700153 which reveals considerable long-term variability .