  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
2.2m telescope of Calar Alto Observatory and the FIES 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 Mp=5.84 +\- 0.05 Mjup 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 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.
