Mass ratio estimates for overcontact binaries using the derivatives of light curves

The photometric mass ratios of eclipsing binaries are usually estimated by light-curve modeling with an iterative method. We propose a new method for estimating the photometric mass ratio of an overcontact binary using the derivatives of a light curve, which provides a reasonable uncertainty value. The method mainly requires only the time interval value between two local extrema found in the third derivative of a light curve, with no need of an iterative procedure. We applied the method to a sample of real overcontact binary data and compared the estimated mass ratios with their spectroscopically determined values. The comparison showed that our estimated mass ratios for $\sim$67% of the samples agreed with their spectroscopic mass ratios within the estimated uncertainties, and the errors for 95% of them are within $\pm 0.1$. Our method should be useful for estimating mass ratios for numerous overcontact eclipsing binaries found with existing and future surveys, as well as for the light-curve analysis of each system.