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We present seventeen high-precision light curves of five transits of theplanet Qatar-2b, obtained from four defocussed 2m-class telescopes. Three ofthe transits were observed simultaneously in the SDSS griz passbands using theseven-beam GROND imager on the MPG/ESO 2.2-m telescope. A fourth was observedsimultaneously in Gunn grz using the CAHA 2.2-m telescope with BUSCA, and in rusing the Cassini 1.52-m telescope. Every light curve shows small anomalies dueto the passage of the planetary shadow over a cool spot on the surface of thehost star. We fit the light curves with the prism+gemc model to obtain thephotometric parameters of the system and the position, size and contrast ofeach spot. We use these photometric parameters and published spectroscopicmeasurements to obtain the physical properties of the system to high precision,finding a larger radius and lower density for both star and planet thanpreviously thought. By tracking the change in position of one starspot betweentwo transit observations we measure the orbital obliquity of Qatar-2 b to be4.3 \pm 4.5 degree, strongly indicating an alignment of the stellar spin withthe orbit of the planet. We calculate the rotation period and velocity of thecool host star to be 11.4 \pm 0.5 d and 3.28 \pm 0.13 km/s at a colatitude of74 degree. We assemble the planet's transmission spectrum over the 386-976 nmwavelength range and search for variations of the measured radius of Qatar-2 bas a function of wavelength. Our analysis highlights a possible H2/He Rayleighscattering in the blue.

Physical properties, starspot activity, orbital obliquity, and transmission spectrum of the Qatar-2 planetary system from multi-colour photometry