  We report the detection of a 0.6 M_J extrasolar planet by WASP-South,
WASP-25b, transiting its solar-type host star every 3.76 days. A simultaneous
analysis of the WASP, FTS and Euler photometry and CORALIE spectroscopy yields
a planet of R_p = 1.22 R_J and M_p = 0.58 M_J around a slightly metal-poor
solar-type host star, [Fe/H] = -0.05 \pm 0.10, of R_{\ast} = 0.92 R_{\odot} and
M_{\ast} = 1.00 M_{\odot}. WASP-25b is found to have a density of \rho_p = 0.32
\rho_J, a low value for a sub-Jupiter mass planet. We investigate the
relationship of planetary radius to planetary equilibrium temperature and host
star metallicity for transiting exoplanets with a similar mass to WASP-25b,
finding that these two parameters explain the radii of most low-mass planets
well.
