HAT-P-10b: A LIGHT AND MODERATELY HOT JUPITER TRANSITING A K DWARF

We report on the discovery of HAT-P-10b, one of the lowest mass (0.487 +/- 0.018 M-J) transiting extrasolar planets (TEPs) discovered to date by transit searches. HAT-P-10b orbits the moderately bright V = 11.89 K dwarf GSC 02340-01714, with a period P = 3.7224747 +/- 0.0000065 days, transit epoch T-c = 2454759.68683 +/- 0.00016 (BJD), and duration 0.1090 +/- 0.0008 days. HAT-P-10b has a radius of 1.005(-0.027)(+0.032) R-J yielding a mean density of 0.594 +/- 0.052 g cm(-3). Comparing these observations with recent theoretical models we find that HAT-P10b is consistent with a similar to 4.5Gyr, almost pure hydrogen and helium gas giant planet with a 10 M-circle plus core. With an equilibrium temperature of T-eq = 1020 +/- 17 K, HAT-P-10b is one of the coldest TEPs. Curiously, its Safronov number theta = 0.053 +/- 0.002 falls close to the dividing line between the two suggested TEP populations.