  We report the discovery of WTS-1b, the first extrasolar planet found by the
WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom
Infrared Telescope (UKIRT) in August 2007. Light curves comprising almost 1200
epochs with a photometric precision of better than 1 per cent to J ~ 16 were
constructed for ~60000 stars and searched for periodic transit signals. For one
of the most promising transiting candidates, high-resolution spectra taken at
the Hobby-Eberly Telescope (HET) allowed us to estimate the spectroscopic
parameters of the host star, a late-F main sequence dwarf (V=16.13) with
possibly slightly subsolar metallicity, and to measure its radial velocity
variations. The combined analysis of the light curves and spectroscopic data
resulted in an orbital period of the substellar companion of 3.35 days, a
planetary mass of 4.01 +- 0.35 Mj and a planetary radius of 1.49+0.16-0.18 Rj.
WTS-1b has one of the largest radius anomalies among the known hot Jupiters in
the mass range 3-5 Mj. The high irradiation from the host star ranks the planet
in the pM class.
