We present precise z -band photometric time series spanning times of transit of the two exoplanets recently discovered by the SuperWASP collaboration .
We find planetary radii of 1.44 \pm 0.08 R _ { J } and 1.04 \pm 0.06 R _ { J } for WASP-1b and WASP-2b , respectively .
These error estimates include both random errors in the photometry and also the uncertainty in the stellar masses .
Our results are 5 times more precise than the values derived from the discovery data alone .
Our measurement of the radius of WASP-2b agrees with previously published models of hot Jupiters that include both a 20- M _ { \earth } core of solid material and the effects of stellar insolation .
In contrast , we find that the models can not account for the large size of WASP-1b , even if the planet has no core .
Thus , we add WASP-1b to the growing list of hot Jupiters that are larger than expected .
This suggests that “ inflated ” hot Jupiters are more common than previously thought , and that any purported explanations involving highly unusual circumstances are disfavored .