Comparison of the observed brightness (a) with the brightness computed from the Doppler images (b). In both cases the observations before the phase jump, which occurred around MJD = 57446, are denoted with plus-signs. The observations after the phase jump are marked with squares. In (a) the dotted line is the harmonic fit to the observations before the phase jump and the dashed line the harmonic fit to the observationsafter the phase jump.