  Whereas thousands of transiting giant exoplanets are known today, only a few
are well characterized with long orbital periods. Here we present KOI-3680b, a
new planet in this category. First identified by the Kepler team as a promising
candidate from the photometry of the Kepler spacecraft, we establish here its
planetary nature from the radial velocity follow-up secured over two years with
the SOPHIE spectrograph at Observatoire de Haute-Provence, France. The combined
analysis of the whole dataset allows us to fully characterize this new
planetary system. KOI-3680b has an orbital period of 141.2417 +/- 0.0001 days,
a mass of 1.93 +/- 0.20 M_Jup, and a radius of 0.99 +/- 0.07 R_Jup. It exhibits
a highly eccentric orbit (e = 0.50 +/- 0.03) around an early G dwarf. KOI-3680b
is the transiting giant planet with the longest period characterized so far
around a single star; it offers opportunities to extend studies which were
mainly devoted to exoplanets close to their host stars, and to compare both
exoplanet populations.
