KELT-7b: A HOT JUPITER TRANSITING A BRIGHT<i>V</i>= 8.54 RAPIDLY ROTATING F-STAR

We report the discovery of KELT-7b, a transiting hot Jupiter with a mass of 1.28 0.18 M J , radius of 1.533 0.047 0.046 -+ R e , 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 Kilodegree Extremely Little Telescope (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 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 spinorbit alignment of = 9 . 7 5 . 2. This is currently the second most rapidly rotating star to have a reflex signal (and thus mass determination) due to a planetary companion measured.