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We report the discovery of KELT-2Ab, a hot Jupiter transiting the bright(V=8.77) primary star of the HD 42176 binary system. The host is a slightlyevolved late F-star likely in the very short-lived "blue-hook" stage ofevolution, with $\teff=6148\pm48{\rm K}$, $\log{g}=4.030_{-0.026}^{+0.015}$ and$\feh=0.034\pm0.78$. The inferred stellar mass is$M_*=1.314_{-0.060}^{+0.063}$\msun\ and the star has a relatively large radiusof $R_*=1.836_{-0.046}^{+0.066}$\rsun. The planet is a typical hot Jupiter withperiod $4.11379\pm0.00001$ days and a mass of $M_P=1.524\pm0.088$\mj\ andradius of $R_P=1.290_{-0.050}^{+0.064}$\rj. This is mildly inflated as comparedto models of irradiated giant planets at the $\sim$4 Gyr age of the system.KELT-2A is the third brightest star with a transiting planet identified byground-based transit surveys, and the ninth brightest star overall with atransiting planet. KELT-2Ab's mass and radius are unique among the subset ofplanets with $V<9$ host stars, and therefore increases the diversity of brightbenchmark systems. We also measure the relative motion of KELT-2A and -2B overa baseline of 38 years, robustly demonstrating for the first time that thestars are bound. This allows us to infer that KELT-2B is an early K-dwarf. Wehypothesize that through the eccentric Kozai mechanism KELT-2B may haveemplaced KELT-2Ab in its current orbit. This scenario is potentially testablewith Rossiter-McLaughlin measurements, which should have an amplitude of$\sim$44 m s$^{-1}$.

KELT-2Ab: A Hot Jupiter Transiting the Bright (V=8.77) Primary Star of a Binary System