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

•B, V, R and I light curves of V1139 Cas have been observed by the 1 m telescope at Weihai Observatory of Shandong University.•The first photometric analysis and long-term period variation investigation are presented.•V1139 Cas is a marginal contact W-type binary and may be a triply system.

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%f=3.6%) with a mass ratio of q=1.583q=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=3.66×10-7dP/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.