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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 RJup versus 0.915+0.046 -0.040 RJup) and much lower density (0.41 ± 0.10 pJup versus 1.26+0.25 -0.21 pJup) and surface gravity (13.4 ± 2.6ms-2 versus 26.4+4.4 -4.0 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. © 2011 ESO.

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Astronomy and Astrophysics