1SWASP J200503.05-343726.5: A high mass ratio eclipsing binary near the period limit

First CCD photometric light curves of the eclipsing binary system 1SWASP J200503.05-343726.5 are presented. Our complete light curves in V, R and I bands using the Bessell filter show an out-of-eclipsing distortion, which means that the components of the system may be active. The preliminary photometric solutions with a cool star-spot are derived by using the 2013 version of the Wilson-Devinney (W-D) code. The photometric solutions suggest that 1SWASP J200503.05-343726.5 is a shallow-contact eclipsing binary(f=9.0%) with a mass ratio of q=1.0705, which is very high for late-type binary systems near the period limit. The primary component is about 230  K hotter than the secondary component. Based on our new CCD eclipse times, the orbital period change was analyzed. According to O−C diagram, the orbital period of the 1SWASP J200503.05-343726.5 shows an increase at a rate of P˙=+5.43×10−8 days year−1. The period increase may be caused by mass transfer from the less massive component to the more massive one. This shallow-contact system may be formed from a detached short-period binary via orbital shrinkage because of dynamical interactions with a third component or by magnetic braking.