A photometric study of the K-type contact binary EI CVn

We present charge-coupled device (CCD) photometry for the short-period K-type binary EI CVn, observed on 2009 February 28 at the Xinglong Station of National Astronomical Observatories, Chinese Academy of Sciences. By using the Wilson-Devinney program, the photometric solution was first deduced from our V R observations. The results show that EI CVn is a W-type weak-contact binary with a mass ratio of q_ph = 0.2834(±0.0010) and an overcontact degree of f = 20.0%(±0.7%). The distorted light curves were modeled by a dark spot on the cool primary component, whose area was up to 1.9% of the area of the primary. Based on the period analysis, it is found that there exists a weak secular decrease at a rate of dP/dt = -3.11(±0.03) × 10^-7d yr^-1, which may be attributed to mass transfer from the primary to the secondary. With mass transfer occurring, the separation between both components will shrink, which may cause the degree of overcontact to increase. Therefore, the weak-contact binary EI CVn may evolve into a deep-contact configuration.