We present high precision transit observations of the exoplanet WASP-21b, obtained with the RISE instrument mounted on 2.0m Liverpool Telescope. A transit model is fitted, coupled with an MCMC routine to derive accurate system parameters. The two new high precision transits allow 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 pm 0.04 Msun which is significantly lower than previously reported (1.01 pm 0.03 Msun). Consequently, we find a lower planetary mass of $0.27 pm 0.01 Mjup$. A lower inclination (87.4 pm 0.3 degrees) is also found for the system than previously reported, resulting in a slightly larger stellar (R_* =1.10 pm 0.03 Rsun) and planetary radius (R_p = 1.14 pm 0.04 Rjup). 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 =2455084.51974 pm 0.00020 (HJD) and P=4.3225060 pm 0.0000031 days. We found no transit timing variations in WASP-21b.
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