A hot Uranus transiting the nearby M dwarf GJ 3470 Detected with HARPS velocimetry. Captured in transit with TRAPPIST photometry

We report on the discovery of GJ 3470 b, a transiting hot Uranus of mass m(p) = 14.0 +/- 1.8 M-circle plus, radius R-p = 4.2 +/- 0.6 R-circle plus and period P = 3.3371 +/- 0.0002 day. Its host star is a nearby (d = 25.2 +/- 2.9 pc) M1.5 dwarf of mass M-star = 0.54 +/- 0.07 M-circle dot and radius R-star = 0.50 +/- 0.06 R-circle dot. The detection was made during 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 GJ 3470b and allowed the determination of its true mass and radius. The star's visible or infrared brightness (V-mag = 12.3, K-mag = 8.0), together with a large eclipse depth D = 0.57 +/- 0.05%, ranks GJ 3470 b among the most suitable planets for follow-up characterizations.