The low mass ratio contact binary system V728 Herculis

•The multi-colour light curve of V728 Her is obtained.•A period analysis is applied to collected times of minimum light.•The light and radial velocity curves of V728 Her are analyzed simultaneously.•We prove that the system is a deep, low mass contact binary.

We present the orbital period study and the photometric analysis of the contact binary system V728 Her. Our orbital period analysis shows that the period of the system increases (dP/dt=1.92×10−7dP/dt=1.92×10−7dyr−1−1) and the mass transfer rate from the less massive component to more massive one is 2.51×10−82.51×10−8 M⊙y−1⊙y−1. In addition, an advanced sinusoidal variation in period can be attributed to the light-time effect by a tertiary component or the Applegate mechanism triggered by the secondary component. The simultaneous multicolor BVR   light and radial velocity curves solution indicates that the physical parameters of the system are M1=1.8M⊙,M1=1.8M⊙,M2=0.28M⊙,M2=0.28M⊙,R1=1.87R⊙,R1=1.87R⊙,R2=0.82R⊙,R2=0.82R⊙,L1=5.9L⊙,L1=5.9L⊙, and L2=1.2L⊙L2=1.2L⊙. We discuss the evolutionary status and conclude that V728 Her is a deep (f = 81%), low mass ratio (q = 0.16) contact binary system.