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Refined physical properties of the HAT-P-13 planetary system

We present photometry of four transits of the planetary system HAT-P-13, obtained using defocussed telescopes. We analyse these, plus nine datasets from the literature, in order to determine the physical properties of the system. The mass and radius of the star are M_A = 1.320 +/- 0.048 +/- 0.039 Msun and R_A = 1.756 +/- 0.043 +/- 0.017 Rsun (statistical and systematic errorbars). We find the equivalent quantities for the transiting planet to be M_b = 0.906 +/- 0.024 +/- 0.018 Mjup and R_b = 1.487 +/- 0.038 +/- 0.015 Rjup, with an equilibrium temperature of 1725 +/- 31 K. Compared to previous results, which were based on much sparser photometric data, we find the star to be more massive and evolved, and the planet to be larger, hotter and more rarefied. The properties of the planet are not matched by standard models of irradiated gas giants. Its large radius anomaly is in line with the observation that the hottest planets are the most inflated, but at odds with the suggestion of inverse proportionality to the [Fe/H] of the parent star. We assemble all available times of transit midpoint and determine a new linear ephemeris. Previous findings of transit timing variations in the HAT-P-13 system are shown to disagree with these measurements, and can be attributed to small-number statistics.