  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/-0.018 rjup and the
planetary mass to be 0.565 +/- 0.054 mjup. These results are consistent with
those reported previously, and are also consistent with theoretical models for
gas giant planets. The mid-transit times are accurate to within 1 min and are
consistent with a constant period. However, the period we derive differs by 2.5
sigma 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.
