Transits and starspots in the WASP-6 planetary system

We present updates to PRISM, a photometric transit-starspot model, and GEMC, a hybrid optimization code combining MCMC and a genetic algorithm. We then present high-precision photometry of four transits in the WASP-6 planetary system, two of which contain a starspot anomaly. All four transits were modelled using PRISM and GEMC, and the physical properties of the system calculated. We find the mass and radius of the host star to be 0.836 +/- 0.063 M-circle dot and 0.864 +/- 0.024 R-circle dot, respectively. For the planet, we find a mass of 0.485 +/- 0.027M(Jup), a radius of 1.230 +/- 0.035 R-Jup and a density of 0.244 +/- 0.014 rho(Jup). These values are consistent with those found in the literature. In the likely hypothesis that the two spot anomalies are caused by the same starspot or starspot complex, we measure the stars rotation period and velocity to be 23.80 +/- 0.15 d and 1.78 +/- 0.20 km s(-1), respectively, at a colatitude of 75.8 degrees. We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is lambda = 7.2 degrees +/- 3.7 degrees, indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect. These results suggest thatWASP-6 b formed at amuch greater distance from its host star and suffered orbital decay through tidal interactions with the protoplanetary disc.