We report the discovery of KELT-7b , a transiting hot Jupiter with a mass of 1.28 \pm 0.18 M _ { J } , radius of 1.533 _ { -0.047 } ^ { +0.046 } R _ { J } , and an orbital period of 2.7347749 \pm 0.0000039 days .
The bright host star ( HD 33643 ; KELT-7 ) is an F-star with V = 8.54 , T _ { eff } = 6789 _ { -49 } ^ { +50 } K , [ Fe/H ] = 0.139 _ { -0.081 } ^ { +0.075 } , and \log { g } = 4.149 \pm 0.019 .
It has a mass of 1.535 _ { -0.054 } ^ { +0.066 } M _ { \sun } , a radius of 1.732 _ { -0.045 } ^ { +0.043 } R _ { \sun } , and is the fifth most massive , fifth hottest , and the ninth brightest star known to host a transiting planet .
It is also the brightest star around which KELT has discovered a transiting planet .
Thus , KELT-7b is an ideal target for detailed characterization given its relatively low surface gravity , high equilibrium temperature , and bright host star .
The rapid rotation of the star ( 73 \pm 0.5 km s ^ { -1 } ) results in a Rossiter-McLaughlin effect with an unusually large amplitude of several hundred m s ^ { -1 } .
We find that the orbit normal of the planet is likely to be well-aligned with the stellar spin axis , with a projected spin-orbit alignment of \lambda = 9.7 \pm 5.2 degrees .
This is currently the second most rapidly rotating star to have a reflex signal ( and thus mass determination ) due to a planetary companion measured .