  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 slightly
evolved late F-star likely in the very short-lived "blue-hook" stage of
evolution, 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 radius
of $R_*=1.836_{-0.046}^{+0.066}$\rsun. The planet is a typical hot Jupiter with
period $4.11379\pm0.00001$ days and a mass of $M_P=1.524\pm0.088$\mj\ and
radius of $R_P=1.290_{-0.050}^{+0.064}$\rj. This is mildly inflated as compared
to 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 by
ground-based transit surveys, and the ninth brightest star overall with a
transiting planet. KELT-2Ab's mass and radius are unique among the subset of
planets with $V<9$ host stars, and therefore increases the diversity of bright
benchmark systems. We also measure the relative motion of KELT-2A and -2B over
a baseline of 38 years, robustly demonstrating for the first time that the
stars are bound. This allows us to infer that KELT-2B is an early K-dwarf. We
hypothesize that through the eccentric Kozai mechanism KELT-2B may have
emplaced KELT-2Ab in its current orbit. This scenario is potentially testable
with Rossiter-McLaughlin measurements, which should have an amplitude of
$\sim$44 m s$^{-1}$.
