Fit to the light curves of the afterglow of GRB 111209A from the beginning of the rebrightening at ≈74 ks to before the “step” at about three days (between the vertical dotted lines). This fit uses GROND data only and is the simplest possible transition between the rising and decaying regimes. The stars represent UVOT data (multiple colours) shifted to the r′ band. This shows that the evolution over the peak is possibly more complicated than our simple model. The decay slope after the peak is a free but shared parameter for all seven bands, whereas the index of the rise is different for each band. See Table 4 for values.