Photometric study and absolute parameters of four short period W UMa binary stars of late spectral classes

We present the first CCD complete light curves for the W Ursae Majoris systems NSVS 870615, NSVS 0912043, NSVS 5049537 and LINEAR 3636597. The observations were performed at the IRIDA South dome of the NAO Rozhen–Bulgaria and they confirm and revise the short-period (0.22-0.25 days) for these objects. In particular two of them show periods near to the cut-off limit. The modeling of the light curves was done using the Wilson–Devinney code and the elements obtained from this analysis are used to compute the physical parameters of the system in order to study its evolutionary status. From a reasonable synthetic fit of the original data we can derive that all components of the systems are of K spectral types (T1< 5000K), one of the secondaries is of M spectral type. All targets are contact binary systems of W-subtype, which is a common feature of K spectral type contact binaries, and two of them, having 1/q > 0.72, belong to the class of the high mass ratio binaries (H/W type). One, with a total mass of about 0.73(M⊙) can be considered as Low Mass Contact Binary system (LMCB). The degrees of contact (fill-out factors) f are shallow for three of our targets, while for NSVS 0912043 the analyzes return an important high value of f = 80.7%. The O’Connell effect in the light curves is well explained by employing star spots on the binary surface, which confirms that the systems are active at present. The four systems are in good thermal contact with ΔT between 170K and 420K. The absolute elements of the systems are estimated and the positions of the components is shown in the logarithmic diagrams.