Rossiter-McLaughlin effect in the radial velocity curve of WASP-21. The transits observed on the nights of September 5, 2011, September 18, 2011, and September 7, 2018 are shown in red, green, and blue, respectively. The median RM model and its 1-σ and 2-σ confidence regions are shown in black and gray. Top panel: anomaly caused by the RM effect. Bottom panel: residuals after removing the median RM model.