  We report the discovery of a hot Jupiter transiting a subgiant star with an
orbital period of 6.87 days thanks to public photometric data from the Kepler
space mission and new radial velocity observations obtained by the SOPHIE
spectrograph. The planet KOI-428b with a radius of 1.17 +/- 0.04 RJup and a
mass of 2.2 +/- 0.4MJup, orbits around a F5IV star with R* = 2.13 +/- 0.06Rsun,
M* = 1.48 +/- 0.06 Msun and Teff = 6510 +/- 100 K. The star KOI-428 is the
largest and the most evolved star discovered so far with a transiting planet.
