WASP-34b: a near-grazing transiting sub-Jupiter-mass exoplanet in a hierarchical triple system

We report the discovery of WASP-34b, a sub-Jupiter-mass exoplanet transiting its 10.4-magnitude solar-type host star (1SWASP J110135.89-235138.4; TYC 6636-540-1) every 4.3177 days in a slightly eccentric orbit (e = 0.038 ± 0.012). We find a planetary mass of 0.59 ± 0.01 M_Jup and radius of 1.22+0.11−0.08 R_Jup. There is a linear trend in the radial velocities of 55±4 m s⁻¹ y⁻¹ indicating the presence of a long-period third body in the system with a mass > 0.45 M_Jup at a distance of > 1.2 AU from the host star. This third-body is either a low-mass star, white dwarf, or another planet. The transit depth ((R_P /R_∗ )² = 0.0126) and high impact parameter (b = 0.90) suggest that this could be the first known transiting exoplanet expected to  undergo grazing transits, but with a confidence of only ∼ 80%.