We present seventeen high-precision light curves of five transits of the planet Qatar-2 b , obtained from four defocussed 2m-class telescopes .
Three of the transits were observed simultaneously in the SDSS g ^ { \prime } r ^ { \prime } i ^ { \prime } z ^ { \prime } passbands using the seven-beam GROND imager on the MPG/ESO 2.2-m telescope .
A fourth was observed simultaneously in Gunn grz using the CAHA 2.2-m telescope with BUSCA , and in r using the Cassini 1.52-m telescope .
Every light curve shows small anomalies due to the passage of the planetary shadow over a cool spot on the surface of the host star .
We fit the light curves with the prism+gemc model to obtain the photometric parameters of the system and the position , size and contrast of each spot .
We use these photometric parameters and published spectroscopic measurements to obtain the physical properties of the system to high precision , finding a larger radius and lower density for both star and planet than previously thought .
By tracking the change in position of one starspot between two transit observations we measure the orbital obliquity of Qatar-2 b to be \lambda = 4.3 ^ { \circ } \pm 4.5 ^ { \circ } , strongly indicating an alignment of the stellar spin with the orbit of the planet .
We calculate the rotation period and velocity of the cool host star to be 11.4 \pm 0.5 d and 3.28 \pm 0.13 km s ^ { -1 } at a colatitude of 74 ^ { \circ } .
We assemble the planet ’ s transmission spectrum over the 386 – 976 nm wavelength range and search for variations of the measured radius of Qatar-2 b as a function of wavelength .
Our analysis highlights a possible H _ { 2 } /He Rayleigh scattering in the blue .