Decomposition of the optical/NIR afterglow of GRB 111209A across the rebrightening episode. UVOT data are the only data available before ≈0.8 days. With the beginning of GROND observations, a steeply rising second component becomes dominant (the two different components are shown as dotted lines for the r′ band), leading to a colour change in the afterglow. The fit ends before the first “step” in the late afterglow.