We present photometry of four transits of the planetary system HAT-P-13, obtained using defocused telescopes. We analyse these, plus nine data sets 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+/-0.039M_{sun}} and R_A_=1.756+/-0.043+/-0.017R_{sun} (statistical and systematic error bars). We find the equivalent quantities for the transiting planet to be M_b=0.906+/-0.024+/-0.018M_Jup_ and R_b_=1.487+/-0.038+/-0.015R_Jup_, with an equilibrium temperature of T'eq=1725+/-31K. 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.