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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 (874 ± 03) 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.  T  0 = 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.

A lower mass for the exoplanet WASP‐21b