WASP-25b: a 0.6 M$_J$ planet in the Southern hemisphere

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 d. 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 plusmn 0.10, of R$_*$= 0.92 R$_⊙$ and M$_*$= 1.00 M$_⊙$. WASP-25b is found to have a density of $$_p$= 0.32 $$_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.