KELT-4Ab: An Inflated Hot Jupiter Transiting the Bright (V ∼ 10) Component of a Hierarchical Triple

We report the discovery of KELT-4Ab, an inflated, transiting Hot Jupiter orbiting the brightest component of a hierarchical triple stellar system. The host star is an F star with {T}_{{eff}} = 6206+/- 75 K, {log}g = 4.108+/- 0.014, [{{Fe}}/{{H}}] = -{0.116}_-0.069^+0.065, {M}_* = {1.201}_-0.061^+0.067 {M}_⊙ , and {R}_* = {1.603}_-0.038^+0.039 {R}_⊙ . The best-fit linear ephemeris is {{BJD}}_{TDB} \=\2456193.29157+/- 0.00021 \quad +\quad E(2.9895936+/- 0.0000048). With a magnitude of V ∼ 10, a planetary radius of {1.699}_-0.045^+0.046 {R}_{{J}}, and a mass of {0.902}_-0.059^+0.060 {M}_{{J}}, it is the brightest host among the population of inflated Hot Jupiters (R_P > 1.5R_J), making it a valuable discovery for probing the nature of inflated planets. In addition, its existence within a hierarchical triple and its proximity to Earth (210 pc) provide a unique opportunity for dynamical studies with continued monitoring with high resolution imaging and precision radial velocities. The projected separation between KELT-4A and KELT-4BC is 328 ± 16 AU and the projected separation between KELT-4B and KELT-4C is 10.30 ± 0.74 AU. Assuming face-on, circular orbits, their respective periods would be 3780 ± 290 and 29.4 ± 3.6 years and the astrometric motions relative to the epoch in this work of both the binary stars around each other and of the binary around the primary star would be detectable now and may provide meaningful constraints on the dynamics of the system.