  We report the discovery of a low-density exoplanet transiting an 11th
magnitude star in the Southern hemisphere. WASP-15b, which orbits its host star
with a period P=3.7520656+-0.0000028d has a mass M_p=0.542+-0.050M_J and radius
R_p=1.428+-0.077R_J, and is therefore the one of least dense transiting
exoplanets so far discovered (rho_p=0.247+-0.035g cm^-3). An analysis of the
spectrum of the host star shows it to be of spectral type around F5, with an
effective temperature T_eff=6300+-100K and [Fe/H]=-0.17+-0.11.
