  We report on the discovery of HAT-P-10b, the lowest mass (0.46 +/- 0.03 MJ)
transiting extrasolar planet (TEP) 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 = 3.7224690 +/- 0.0000067 d, transit epoch Tc = 2454729.90631 +/-
0.00030 (BJD) and duration 0.1100 +/- 0.0015 d. HAT-P-10b has a radius of 1.05
+(0.05)-(0.03) RJ yielding a mean density of 0.498+/-0.064 g cm^-3 . Comparing
these observations with recent theoretical models we find that HAT-P-10 is
consistent with a ~4.5 Gyr, coreless, pure hydrogen and helium gas giant
planet. With an equilibrium temperature of Teq = 1030 +(26)-(19)K, HAT-P-10b is
one of the coldest TEPs. Curiously, its Safronov number Theta = 0.047 +/- 0.003
falls close to the dividing line between the two suggested TEP populations.

Astrophysics