  We report the discovery of WASP-21b, a new transiting exoplanet discovered by
the Wide Angle Search for Planets (WASP) Consortium and established and
characterized with the FIES, SOPHIE, CORALIE and HARPS fiber-fed echelle
spectrographs. A 4.3-d period, 1.1% transit depth and 3.4-h duration are
derived for WASP-21b using SuperWASP-North and high precision photometric
observations at the Liverpool Telescope. Simultaneous fitting to the
photometric and radial velocity data with a Markov Chain Monte Carlo procedure
leads to a planet in the mass regime of Saturn. With a radius of 1.07 R_Jup and
mass of 0.30 M_Jup, WASP-21b has a density close to 0.24 rho_Jup corresponding
to the distribution peak at low density of transiting gaseous giant planets.
With a host star metallicity [Fe/H] of -0.46, WASP-21b strengthens the
correlation between planetary density and host star metallicity for the five
known Saturn-like transiting planets. Furthermore there are clear indications
that WASP-21b is the first transiting planet belonging to the thick disc.
