Orbital Solutions and Absolute Elements of the Short-Period Eclipsing Binary ES Librae

We have obtained new differential UBV photoelectric photometry and radial velocities of both components of the short-period eclipsing binary ES Lib. The system has a circular orbit with a period of 0.883040928 days and is seen at an inclination of 70.1 degrees degrees. With the Wilson-Devinney analysis program, we obtained a simultaneous solution of our photometric and spectroscopic observations that resulted in masses of M-1=2.30 +/-+/- 0.03M and M-2=0.97 +/-+/- 0.01M and the equal-volume radii of R-1=2.69 +/-+/- 0.02R and R-2=1.83 +/-+/- 0.01R for the primary and secondary, respectively. The secondary is oversized and overluminous for its mass. The effective temperatures of the primary and secondary are 8500K (fixed) and 5774 +/-+/- 57K, respectively. Despite the very large temperature difference, our photometric and spectroscopic data indicate that ES Lib is not semidetached but rather require it to be in an overcontact state, where both components exceed their critical Roche lobes. Given its nonthermal equilibrium state, if the overcontact solution correctly characterizes the system, the change from being semidetached to overcontact may have occurred recently. While the asymmetry of the light curves can be modeled well with a large, hot starspot or a large, cool one on the secondary component, we prefer the latter interpretation because cool spots are a typical feature on many contact binaries.