  We report the discovery of WASP-3b, the third transiting exoplanet to be
discovered by the WASP and SOPHIE collaboration. WASP-3b transits its host star
USNO-B1.0 1256-0285133 every 1.846834+-0.000002 days. Our high precision
radial-velocity measurements present a variation with amplitude characteristic
of a planetary-mass companion and in-phase with the light-curve. Adaptive
optics imaging shows no evidence for nearby stellar companions, and
line-bisector analysis excludes faint, unresolved binarity and stellar activity
as the cause of the radial-velocity variations. We make a preliminary
spectroscopic analysis of the host star finding it to have Teff = 6400+-100 K
and log g = 4.25+-0.05 which suggests it is most likely an unevolved main
sequence star of spectral type F7-8V. Our simultaneous modelling of the transit
photometry and reflex motion of the host leads us to derive a mass of 1.76
+0.08 -0.14 M_J and radius 1.31 +0.07-0.14 R_J for WASP-3b. The proximity and
relative temperature of the host star suggests that WASP-3b is one of the
hottest exoplanets known, and thus has the potential to place stringent
constraints on exoplanet atmospheric models.
