  We report on the discovery of GJ3470b, a transiting hot Uranus of mass m_p =
14.0+-1.8 Mearth, radius R_p = 4.2+-0.6 Rearth and period P=3.3371+-0.0002 day.
Its host star is a nearby (d=25.2+-2.9pc) M1.5 dwarf of mass M_s=0.54+-0.07
Msol and radius R_s=0.50+-0.06 Rsol. The detection originates from a
radial-velocity campaign with HARPS that focused on the search for short-period
planets orbiting M dwarfs. Once the planet was discovered and the
transit-search window narrowed to about 10% of an orbital period, a photometric
search started with TRAPPIST and quickly detected the ingress of the planet.
Additional observations with TRAPPIST, EulerCam and NITES definitely confirmed
the transiting nature of GJ3470b and allow for the determination of its true
mass and radius. The star's visible or infrared brightness (V=12.3, K=8.0 mag),
together with a large eclipse depth D=0.57+-0.05%, ranks GJ3470b among the most
favorable planets for follow-up characterizations.
