Lomb–Scargle periodogram (Lomb 1976; Scargle 1982) of the WASP data for HD 21 520 from 2006 to 2014 combined (top). There is no significant periodicity. The horizontal lines are at the estimated 10 per cent and 1 per cent false-alarm levels. The peaks near 30 d are the residual effects of moonlight propagating through the pipeline. The red tick marks the orbital period of the planet.