Afterglow light curve of GRB 100621A as observed with Swift in X-rays (top) and GROND in its seven filter bands (bottom). The J-band data points at 14 ks are from SOFI imaging, and the HK_s-band data at 20 ks from a GROND observation in morning twilight at which the J-band was already saturated by the rising Sun. The seven vertical lines mark the times at which spectral energy distributions have been extracted (see text and Fig. 3).