The transit light curve project.: I.: Four consecutive transits of the exoplanet XO-1b

We present RIz photometry of four consecutive transits of the newly discovered exoplanet XO-1b. We improve on the estimates of the transit parameters, finding the planetary radius to be R-P 1.184(-0.018)(+0.028)R(J), and the stellar radius to be R-star 0.928(-0.013)(+0.018) R-circle dot, assuming a stellar mass of M-star (1.00 +/- 0.03) M-circle dot. The uncertainties in the planetary and stellar radii are dominated by the uncertainty in the stellar mass. These uncertainties increase by a factor of 2-3 if a more conservative uncertainty of 0.10 M-circle dot is assumed for the stellar mass. Our estimate of the planetary radius is smaller than that reported by McCullough and coworkers, and the resulting estimate for the mean density of XO-1b is intermediate between that of the low-density planet HD 209458b and the higher density planets TrES-1 and HD 189733b. The timings of the transits have an accuracy ranging from 0.2 to 2.5 minutes and are marginally consistent with a uniform period.