We report the discovery of HAT-P-67b , a hot-Saturn transiting a rapidly rotating F-subgiant .
HAT-P-67b has a radius of R _ { p } = 2.085 _ { -0.071 } ^ { +0.096 } R _ { J } , orbiting a M _ { * } = 1.642 _ { -0.072 } ^ { +0.155 } M _ { \odot } , R _ { * } = 2.546 _ { -0.084 } ^ { +0.099 } R _ { \odot } host star in a \sim 4.81 -day period orbit .
We place an upper limit on the mass of the planet via radial velocity measurements to be M _ { p } < 0.59 M _ { J } , and lower limit of > 0.056 M _ { J } 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 _ { \star } } = 35.8 \pm 1.1 km s ^ { -1 } , 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 ^ { \circ } , 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 .