  We report the discovery of HAT-P-67b, a hot-Saturn transiting a rapidly
rotating F-subgiant. HAT-P-67b has a radius of Rp = 2.085 -0.071/+0.096 RJ,,
orbiting a M* = 1.642 -0.072/+0.155 Msun, R* = 2.546 -0.084/+0.099 Rsun host
star in a ~4.81-day period orbit. We place an upper limit on the mass of the
planet via radial velocity measurements to be Mp < 0.59 MJ, and lower limit of
> 0.056 MJ by limitations on Roche lobe overflow. Despite being a subgiant, the
host star still exhibits relatively rapid rotation, with a projected rotational
velocity of v sin I* = 35.8 +/- 1.1 km/s, making it difficult to precisely
determine the mass of the planet using radial velocities. We validated
HAT-P-67b via two Doppler tomographic detections of the planetary transit,
which eliminated potential eclipsing binary blend scenarios. The Doppler
tomographic observations also confirmed that HAT-P-67b has an orbit that is
aligned to within 12 degrees, in projection, with the spin of its host star.
HAT-P-67b receives strong UV irradiation, and is amongst the one of the lowest
density planets known, making it a good candidate for future UV transit
observations to search for an extended hydrogen exosphere.
