We present RIz photometry of four consecutive transits of the newly discovered exoplanet XO-1b .
We improve upon the estimates of the transit parameters , finding the planetary radius to be R _ { P } = 1.184 _ { -0.018 } ^ { +0.028 } ~ { } R _ { Jup } , and the stellar radius to be R _ { S } = 0.928 _ { -0.013 } ^ { +0.018 } ~ { } R _ { \odot } , assuming a stellar mass of M _ { S } = 1.00 \pm 0.03 ~ { } M _ { \odot } .
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 _ { \odot } is assumed for the stellar mass .
Our estimate of the planetary radius is smaller than that reported by McCullough et al .
( 23 ) and yields a mean density that is comparable to that of 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 .