Magnetic activity and orbital period variation of the eclipsing binary KV Gem

•We present new CCD BVRI light curves of KV Gem in 2010 and 2011.•Photometric solutions of KV Gem are obtained and starspot parameters are also derived.•KV Gem exists a cyclic variation overlaying a continuous period decrease.•This cyclic variation maybe caused by the light-time effect or magnetic cycle.

This paper presents new CCD BVRI light curves of a neglected eclipsing binary KV Gem. Our new light curves were obtained in 2010 and 2011 at the Xinglong station of the National Astronomical Observatories, China. By analyzing all available light minimum times, we derived an update ephemeris and found there existed a cyclic variation overlaying a continuous period decrease. This kind of cyclic variation may probably be attributed to the light-time effect via the presence of an unseen third body or magnetic activity cycle. The long-term period decrease suggests that KV Gem is undergoing a mass transfer from the secondary component to the primary component at a rate of 3.4(0.3)×(10-7M⊙/year for period decrease and a third body (10.3±0.210.3±0.2 years), and 5.5(0.6)×10-7M⊙/year for decrease and magnetic cycle (8.8±0.18.8±0.1 years). By analyzing the light curves in 2011, photometric solutions and starspots parameters of the system are obtained using Wilson–Devinney program. Based on the photometric solution in 2011, we still could use the spot model to explain successfully our light curves in 2010 and three published light curves. Comparing the starspot longitudes and factors, KV Gem are variable on a long time scale of about years. For the data of KV Gem, the brightness vary with time around phases 0, 0.25, 0.5, and 0.75, which means that there is a possible photospheric active evolution. More data are needed to monitor to detect stellar cycle of KV Gem. For chromospheric activity of KV Gem, we found strong absorption in the observed Hβ,HγHβ,Hγ, and Ca II H & K spectra, and no obvious emission.