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Description
We present the discovery of HAT-P-57b, a P=2.4653 day transiting planet around a V=10.465+/-0.029mag, T_eff_=7500+/-250K main sequence A8V star with a projected rotation velocity of vsini=102.1+/-1.3km/s. We measure the radius of the planet to be R=1.413+/-0.054R_J_ and, based on RV observations, place a 95% confidence upper limit on its mass of M<1.85M_J_. Based on theoretical stellar evolution models, the host star has a mass and radius of 1.47+/-0.12M_{sun}_ and 1.500+/-0.050R_{sun}_, respectively. Spectroscopic observations made with Keck-I/HIRES during a partial transit event show the Doppler shadow of HAT-P-57b moving across the average spectral line profile of HAT-P-57, confirming the object as a planetary system. We use these observations, together with analytic formulae that we derive for the line profile distortions, to determine the projected angle between the spin axis of HAT-P-57 and the orbital axis of HAT-P-57b. The data permit two possible solutions, with -16.7{deg}<{lambda}<3.3{deg} or 27.6{deg}<{lambda}<57.4{deg} at 95% confidence, and with relative probabilities for the two modes of 26% and 74%, respectively. Adaptive optics imaging with MMT/Clio2 reveals an object located 2.7" from HAT-P-57 consisting of two point sources separated in turn from each other by 0.22". The H- and L'-band magnitudes of the companion stars are consistent with their being physically associated with HAT-P-57, in which case they are stars of mass 0.61+/-0.10M_{sun}_ and 0.53+/-0.08M_{sun}_. HAT-P-57 is the most rapidly rotating star, and only the fourth main sequence A star, known to host a transiting planet.
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