  We report the discovery of HAT-P-24b, a transiting extrasolar planet orbiting
the moderately bright V=11.818 F8 dwarf star GSC 0774-01441, with a period P =
3.3552464 +/- 0.0000071 d, transit epoch Tc = 2455216.97669 +/- 0.00024
(BJD_UTC), and transit duration 3.653 +/- 0.025 hours. The host star has a mass
of 1.191 +/- 0.042 Msun, radius of 1.317 +/- 0.068 Rsun, effective temperature
6373 +/- 80 K, and a low metallicity of [Fe/H] = -0.16 +/- 0.08. The planetary
companion has a mass of 0.681 +/- 0.031 MJ, and radius of 1.243 +/- 0.072 RJ
yielding a mean density of 0.439 +/- 0.069 g cm-3 . By repeating our global
fits with different parameter sets, we have performed a critical investigation
of the fitting techniques used for previous HAT planetary discoveries. We find
that the system properties are robust against the choice of priors. The effects
of fixed versus fitted limb darkening are also examined. HAT-P-24b probably
maintains a small eccentricity of e = 0.052 +0.022 -0.017, which is accepted
over the circular orbit model with false alarm probability 5.8%. In the absence
of eccentricity pumping, this result suggests HAT-P-24b experiences less tidal
dissipation than Jupiter. Due to relatively rapid stellar rotation, we estimate
that HAT-P-24b should exhibit one of the largest known Rossiter-McLaughlin
effect amplitudes for an exoplanet (deltaVRM ~ 95 m/s) and thus a precise
measurement of the sky-projected spin-orbit alignment should be possible.
