WASP-39b: a highly inflated Saturn-mass planet orbiting a late G-type star

We present the discovery of WASP-39b, a highly inflated transiting Saturn-mass planet orbiting a late G-type dwarf star with a period of $4.055259 \pm 0.000008$d,TransitEpochT${0}=2455342.9688\pm0.0002$,(HJD), of duration $0.1168\pm 0.0008$,d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of $\text{mpl}=0.28\pm 0.03\text{mj}$ and a radius of $\text{rpl}=1.27\pm 0.04\text{rj}$, resulting in a mean density of $0.14\pm 0.02\text{rhoj}$. The stellar parameters are mass $\text{mstar}=0.93\pm 0.03\text{msun}$, radius $\rstar = 0.895\pm 0.23,\rsun$,andage$9^{+3}{-4}$,Gyr. Only WASP-17b and WASP-31b have lower densities than WASP-39b, although they are slightly more massive and highly irradiated planets. From our spectral analysis, the metallicity of WASP-39 is measured to be \feh,$=-0.12\pm 0.1$,dex, and we find the planet to have an equilibrium temperature of $1116_{-32}^{+33}$,K,. Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.