We present photometry of the G0 star HAT-P-1 during six transits of its close-in giant planet , and we refine the estimates of the system parameters .
Relative to Jupiter ’ s properties , HAT-P-1b is 1.20 \pm 0.05 times larger and its surface gravity is 2.7 \pm 0.2 times weaker .
Although it remains the case that HAT-P-1b is among the least dense of the known sample of transiting exoplanets , its properties are in accord with previously published models of strongly irradiated , coreless , solar-composition giant planets .
The times of the transits have a typical accuracy of 1 min and do not depart significantly from a constant period .