A much lower density for the transiting extrasolar planet WASP-7

We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 +/- 0.093 R(Jup) versus 0.915(-0.040)(+0.046) R(Jup)) and much lower density (0.41 +/- 0.10 rho(Jup) versus 1.26(-0.21)(+0.25) rho(Jup)) and surface gravity (13.4 +/- 2.6 ms(-2) versus 26.4(-4.0)(+4.4) ms(-2)) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass-radius and period-gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.