2009AJ....137.4911F

We present photometry of six transits of the exoplanet XO-2b. By combining the light-curve analysis with theoretical isochrones to determine the stellar properties, we find the planetary radius to be 0.996^+0.031_–0.018R_Jup and the planetary mass to be 0.565±0.054 M_Jup. These results are consistent with those reported previously, and are also consistent with theoretical models for gas giant planets. The midtransit times are accurate to within 1 minute and are consistent with a constant period. However, the period we derive differs by 2.5σ from the previously published period. More data are needed to tell whether the period is actually variable (as it would be in the presence of an additional body) or if the timing errors have been underestimated.