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Description
We report the discovery of WASP-117b, the first planet with a period beyond 10 days found by the WASP survey. The planet has a mass of M_p_=0.2755(+/- 0.0089)M_jup_, a radius of R_p_=1.021(-0.065/+0.076)R_jup_ and is in an eccentric (e=0.302(+/-0.023)), P=10.02165(+/-0.00055)d orbit around a main-sequence F9 star. The host star's brightness (V=10.15mag) makes WASP-117 a good target for follow-up observations, and with a periastron planetary equilibrium temperature of T_eq_=1225(-39/+36)K and a low planetary mean density (rho_p_=0.259(-0.048/+0.054)rho_jup_) it is one of the best targets for transmission spectroscopy among planets with periods around 10 days. From a measurement of the Rossiter-McLaughlin effect, we infer a projected angle between the planetary orbit and stellar spin axes of beta=-44(+/-11){deg}, and we further derive an orbital obliquity of psi=69.6(+4.7/-4.1){deg}. Owing to the large orbital separation, tidal forces causing orbital circularization and realignment of the planetary orbit with the stellar plane are weak, having had little impact on the planetary orbit over the system lifetime. WASP-117b joins a small sample of transiting giant planets with well characterized orbits at periods above ~8-days.
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