The 0.5

We report the discovery of WASP-13b, a low-mass $ M_{\rm p} = 0.46 ^{+ 0.06 }_{- 0.05}~M_J$ transiting exoplanet with an orbital period of 4.35298 ± 0.00004 days. The transit has a depth of 9 mmag, and although our follow-up photometry does not allow us to constrain the impact parameter well (0 < b < 0.46), with radius in the range Rp ~ 1.06-1.21 RJ the location of WASP-13b in the mass-radius plane is nevertheless consistent with H/He-dominated, irradiated, low core mass and core-free theoretical models. The G1V host star is similar to the Sun in mass $(M_{*} = 1.03^{+0.11}_ {- 0.09}~M_{\odot})$ and metallicity ([M/H] = 0.0±0.2), but is possibly older ($8.5^{+ 5.5 }_{\rm -4.9}$ Gyr)