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Description
We present the multi-band photometric and spectroscopic study of an over-contact binary system, EPIC211957146. The light curves exhibit a variable O' Connell effect, confirmed from our observational data and the Kepler K2 data. The best photometric solution incorporating a dark spot over the primary component unveils that the system has a low-mass ratio (q~0.17) and a high inclination (i~85{deg}). To confirm the solution and constrain the uncertainty, Monte-Carlo simulations are performed and the results are reported. Based on the O-C diagram analysis, we see that the variable shows a period increase at the rate of dP/dt~1.06x10^-6^days/yr, which is higher than the theoretically predicted value. Presence of a third body having a period of ~16.23years is evident from the O-C diagram. No filled-in effect is observed in the H{alpha} line, while the effect is vividly present in the Na line. From the Kepler K2 data, we found that the primary and secondary minima exhibit an anti-correlated O-C variation followed by an erratic behavior. This is possibly caused by the longitudinal motion of the spot, and hence, we set a lower limit of ~40days for the spot modulation. We also observe a possibly associated photometric difference in the primary depth by comparing our light curves with Kepler K2 normalized light curves. This system has a low-mass ratio and a high fill-out factor, and, theoretically, such a physical configuration would lead to a merger.
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