We discuss the orbital period changes of the Algol semi-detached eclipsing binary DI Peg by con- structing the (O−C) residual diagram via using all the available precise minima times. We conclude that the period variation can be explained by a sine–like variation due to the presence of a third body orbiting the binary, together with a long-term orbital period increase (dP/dt = 0.17 sec/century) that can be in- terpreted to be due to mass transfer from the evolved secondary component (of rate 1.52×10−8M⊙/yr) to the primary one. The detected low-mass third body (M3 min. = 0.22±0.0006 M⊙) is responsible for a periodic variation of about 55 years light time effect. We have determined the orbital parameters of the third component which show a considerable eccentricity e3 = 0.77± 0.07 together with a longitude of periastron ω3 = 300° ± 10°.

The authors have assembled a sample of 80 W UMa binary systems (42 W-subtype and 38 A-subtype) whose light curves have all been solved by means of the recent W-D code and combined with up-to-date radial velocity solutions. The absolute parameters (masses, radii and luminosities) have been derived (without any constraint on the physical parameters). The main results of this paper are: (1) the mass-luminosity relations for both W&A-subtypes. as well as for all W UMa contact binaries have been shown, (2) the mass-radius relations have been found for both subtypes, (3) some remarks on the evolution status have been presented.