A new photometric study of the binary U Pegasi

Abstract

Over four hundred photoelectric observations of U Peg in B and V were secured with a 0.6M reflector at Beijing Observatory in 1978. Four times of minima were determined. A period study of the times of minima from 1896 to 1980 was performed. The system was found to have a secular period decrease, Δp/ p of −1.32 ¢ 10 −1 or −4.16 × 10 −3 sec/yr, as well as a short term (17 years) sinusoidal oscillation with a semi-amplitude of 0.00323 day. It is suggested that oscillating term is caused by the light-time effect of an unseen third body. The third body may be a M6 main sequence star with a mass of 0.16 M ⊙. The longterm secular change in period may be associated with slow mass transfer. The analysis of the 1978 light curves together with the 1958 light curves of Binnendijk suggest that the system U Peg has an overcontact configuration of about 9%. It has the characteristics of a W-type W UMa system. The photometric mass ratio, m 2 m 1 , is between 3 and 2.5. If we correct the Struve et al. γ-velocity from 0 km/sec to about −40 km/sec the estimated spectroscopic mass ratio would agree with the photoelectric value. Based on the above assumption the absolute dimensions of U Peg are of 0.6 and 1.8 M ⊙ and of 0.8 and 1.4 R ⊙, for components 1 and 2 respectively. The physical dimensions indicate that the components are main sequence stars.