We present the discovery of a hot-Jupiter transiting the V = 9.23 mag main-sequence A-star KELT-17 ( BD+14 1881 ) .
KELT-17b is a 1.31 _ { -0.29 } ^ { +0.28 } M _ { \mathrm { J } } , 1.645 _ { -0.055 } ^ { +0.060 } R _ { \mathrm { J } } hot-Jupiter in a 3.08 day period orbit misaligned at -115.9 \pm 4.1 deg to the rotation axis of the star .
The planet is confirmed via both the detection of the radial velocity orbit , and the Doppler tomographic detection of the shadow of the planet during two transits .
The nature of the spin-orbit misaligned transit geometry allows us to place a constraint on the level of differential rotation in the host star ; we find that KELT-17 is consistent with both rigid-body rotation and solar differential rotation rates ( \alpha < 0.30 at 2 \sigma significance ) .
KELT-17 is only the fourth A-star with a confirmed transiting planet , and with a mass of 1.635 _ { -0.061 } ^ { +0.066 } M _ { \odot } , effective temperature of 7454 \pm 49 K , and projected rotational velocity v \sin I _ { * } = 44.2 _ { -1.3 } ^ { +1.5 } \mathrm { km s } ^ { -1 } ; it is amongst the most massive , hottest , and most rapidly rotating of known planet hosts .