Global parameters of the totally-eclipsing W UMa stars NSVS 6673994, NSVS 4316778, PP Lac and NSVS 1926064
Photometric observations of four totally-eclipsing W UMa binaries, NSVS 6673994, NSVS 4316778, PP Lac and NSVS 1926064, are presented. Their global parameters were determined from the light curve solutions and GAIA distances. The main results are as follows: (i) NSVS 6673994, NSVS 4316778 and PP Lac are of W subtype while NSVS 1926064 is of A subtype; (ii) The mass ratios of NSVS 4316778 and NSVS 1926064 are close to the lower mass-ratio limit; (iii) NSVS 4316778 has double contact configuration while NSVS 1926064 is in deep contact; (iv) The changes of the PP Lac period seem cyclic and are accompanied with episodes of essentially constant value; (v) The relations between the global parameters of the four W UMa stars differ considerably from those of Main Sequence (MS) stars. The deviations of the radii, temperatures and luminosities of the stellar components from those of MS stars with the same masses are bigger for the two targets with extremely small mass ratio.