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°.

ABSTRACT
1. INTRODUCTION
2. PERIOD VARIATION STUDY
    2.1 Light Elements
    2.2 Previous Light Curve Studies
    2.3 The O − C Diagram
3. CONCLUSIONS
REFERENCES

• M. A. HANNA(National Research Institute of Astronomy and Geophysics (NRIAG))
• S. M. AMIN(National Research Institute of Astronomy and Geophysics (NRIAG))