2011MNRAS.416.2593B

We present high-precision transit observations of the exoplanet WASP-21b, obtained with the Rapid Imager to Search for Exoplanets instrument mounted on the 2.0-m Liverpool Telescope. A transit model is fitted, coupled with a Markov chain Monte Carlo routine, to derive accurate system parameters. The two new high-precision transits allow us to estimate the stellar density directly from the light curve. Our analysis suggests that WASP-21 is evolving off the main sequence which led to a previous overestimation of the stellar density. Using isochrone interpolation, we find a stellar mass of 0.86 ± 0.04 M_☉, which is significantly lower than previously reported (1.01 ±0.03 M_☉). Consequently, we find a lower planetary mass of 0.27 ±0.01 M_Jup. A lower inclination (87°4±0°3) is also found for the system than previously reported, resulting in a slightly larger stellar (R_*= 1.10±0.03 R_☉) and planetary radius (R_p= 1.14±0.04 R_Jup). The planet radius suggests a hydrogen/helium composition with no core which strengthens the correlation between planetary density and host star metallicity. A new ephemeris is determined for the system, i.e. T₀= 245 5084.519 74±0.000 20 (HJD) and P= 4.322 5060±0.000 0031 d. We found no transit timing variations in WASP-21b.