a) Normalized raw light curve of CoRoT-21 during the observation time from MJD 54 396 to MJD 54 529. Strong variations in stellar flux are obvious. b) Light curve after removing a common trend as computed from a subset of 200 light curves and after harmonic filtering. c) Phase diagram of the trend-filtered light curve at period 2.7472 days. The phase is divided into 300 bins with mean flux and standard deviation displayed for each bin.