A possible expanding component in the extreme mass ratio deep contact binary V710 Monocerotis

• The masses and the radii of each component for V710 Mon are determined.
• The age and distance of V710 Mon are estimated.
• We found that V710 Mon may contain a post-main-sequence component.
• The expansion of the primary component may cause the orbital period increase.

We present CCD light curves of V710 Mon obtained in 2007 and 2013. By using the 2010 version of the W-D code, it is found that the star is a deep (59.5–62.7%) contact binary system, with an extreme mass ratio (0.143–0.183). The period of the system increases with a rate dP/dt=+1.95(±0.06)×10-7dP/dt=+1.95(±0.06)×10-7 days/year. We propose that the system contains a post-main-sequence component which may be in an expanding phase. The time scale of the orbital increase nearly equals to the time for the post-main-sequence star to evolve into a subgiant. The age of the primary is estimated to be 5.34 Gyr.