The Period Analysis of the Hierarchical System DI Peg

The existence of an additional body around a binary system can be detected by the help of the light-travel time effect. Due to the motions of binary and the companion stars around the common mass center of the ternary system, the light-time effect produces an irregularity on the eclipse timings. Monitoring the variations in these timings, sub-stellar or planet companions orbiting around the binary system can be identified. In this paper, additional bodies orbiting the Algol type binary DI Peg are examined by using the archival eclipse timings including our CCD data observed at the Ankara University Kreiken Observatory. More than five hundred minimum times equivalent to around nine decades are employed to identify the orbital behavior of the binary system. The best fit to the timings shows that the orbital period of DI Peg has variation due to an integration of two sinusoids with the periods of $P_3 = 49.50\pm0.36$ yr and $P_4 = 27.40\pm0.24$ yr. The orbital change is thought to be most likely due to the existence of two M-type red dwarf companions with the masses of $M_3 =0.213 \pm 0.021$ M$_\odot$ and $M_4 = 0.151 \pm 0.008$ M$_\odot$, assuming that the orbits of additional bodies are co-planar with the orbit of the binary system. Also, the residuals of two sinusoidal fits still seem to show another modulation with the period of roughly $P = 19.5$ yr. The origin of this modulation is not clear and more observational data are required to reveal if the periodicity is caused by another object gravitationally bounded to the system.