Kepler-432 b: a massive warm Jupiter in a 52-day eccentric orbit transiting a giant star

We study the Kepler object Kepler-432, an evolved star ascending the red giant branch. By deriving precise radial velocities from multi epoch high resolution spectra of Kepler-432 taken with the CAFE spectrograph at the 2.2 m telescope of Calar Alto Observatory and the PIES spectrograph at the Nordic Optical Telescope of Roque de Los Muchachos Observatory, we confirm the planetary nature of the object Kepler-432 b, which has a transit period of 52 days. We find a planetary mass of M-p = 5.84 +/- 0.05 M-Jup and a high eccentricity of e = 0.478 +/- 0.004, With a semi-major axis of a = 0.303 +/- 0.007 AU, Kepler-432 b is the first bona fide warm Jupiter detected to transit a giant star. We also find a radial velocity linear trend of gamma = 0.44 +/- 0.04 m s(-1) d(-1), which suggests the presence of a third object in the system. Current models of planetary evolution in the post-main-sequence phase predict that Kepler-432 b will be most likely engulfed by its host star before the latter reaches the tip of the red giant branch.