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 , Transit Epoch T _ { 0 } = 2455342.9688 \pm 0.0002 ( 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 M _ { pl } = 0.28 \pm 0.03 M _ { J } and a radius of R _ { pl } = 1.27 \pm 0.04 R _ { J } , resulting in a mean density of 0.14 \pm 0.02 \rho _ { J } .
The stellar parameters are mass M _ { \star } = 0.93 \pm 0.03 M _ { \odot } , radius R _ { \star } = 0.895 \pm 0.23 R _ { \odot } , and age 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 [ Fe/H ] = -0.12 \pm 0.1 dex , and we find the planet to have an equilibrium temperature of 1116 ^ { +33 } _ { -32 } K .
Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets . Spectroscopic and photometric data are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/