The orbital period changes of theWUMa eclipsing binary AU Ser are studied using the (O−C) method. We conclude that the period variation is due to mass transfer from the primary star to the secondary one at a very low and decreasing rate dP/dt = −8.872× 10−8, superimposed on the sinusoidal variation due to a third body orbiting the binary with period 42.87 ± 3.16 years, orbital eccentricity e = 0.52±0.12 and a longitude of periastron passage ! = 133◦.7±15. On studying the magnetic activity, we have concluded that the Applegate mechanism failed to describe the cycling variation of the (O − C) diagram of AU Ser.

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°.