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.

Abstract
1. INTRODUCTION
2. PERIOD VARIATION
    2.1. Mass Transfer
    2.2. Light Time Effect
    2.3. Magnetic Activity and Star Spots
3. DISCUSSION AND CONCLUSIONS
REFERENCES

• S. M. Amin(National Research Institute of Astronomy and Geophysics (NRIAG)) Corresponding author