  We report the discovery by the WASP transit survey of a giant planet in a
close orbit (0.0295+-0.0009 AU) around a moderately bright (V=11.6, K=10) G9
dwarf (0.89+-0.08 M_sun, 0.84+-0.03 R_sun) in the Southern constellation
Eridanus. Thanks to high-precision follow-up photometry and spectroscopy
obtained by the telescopes TRAPPIST and Euler, the mass and size of this
planet, WASP-50b, are well constrained to 1.47+-0.09 M_jup and 1.15+-0.05
R_jup, respectively. The transit ephemeris is 2455558.6120 (+-0.0002) + N x
1.955096 (+-0.000005) HJD_UTC. The size of the planet is consistent with basic
models of irradiated giant planets. The chromospheric activity (log R'_HK =
-4.67) and rotational period (P_rot = 16.3+-0.5 days) of the host star suggest
an age of 0.8+-0.4 Gy that is discrepant with a stellar-evolution estimate
based on the measured stellar parameters (rho_star = 1.48+-0.10 rho_sun, Teff =
5400+-100 K, [Fe/H]= -0.12+-0.08) which favours an age of 7+-3.5 Gy. This
discrepancy could be explained by the tidal and magnetic influence of the
planet on the star, in good agreement with the observations that stars hosting
hot Jupiters tend to show faster rotation and magnetic activity (Pont 2009;
Hartman 2010). We measure a stellar inclination of 84 (-31,+6) deg,
disfavouring a high stellar obliquity. Thanks to its large irradiation and the
relatively small size of its host star, WASP-50b is a good target for
occultation spectrophotometry, making it able to constrain the relationship
between hot Jupiters' atmospheric thermal profiles and the chromospheric
activity of their host stars proposed by Knutson et al. (2010).
