The first photometric analysis and period investigation of the W UMa type binary system V1139 Cas.

V1139 Cas, which is a very short period W UMa type binary star, was a neglected object since its discovery. BVRI light curves of this system observed using the 1 m telescope at Weihai Observatory of Shandong University are presented and are analyzed using the Wilson–Devinney code. It is discovered that V1139 Cas is a shallow contact binary system ( f = 3.6 % ) with a mass ratio of q = 1.583 . By using all available times of minimum light, the orbital period variation is studied for the first time. We found that the orbital period has varied by a combination of an downward parabola and a sinusoid. The downward parabola means continuous period decrease at a rate of dP / dt = 3.66 × 10 - 7 d yr −1 and may be caused by angular momentum loss via stellar wind. The sinusoidal variation with a period of 12.8 yr and a semi-amplitude of 0.0064 days can most likely be interpreted as the light travel time effect due to the existence of an unseen tertiary companion. [ABSTRACT FROM AUTHOR]