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We present optical and near-infrared photometry of the afterglow of the longGamma-Ray Burst GRB 070802 at redshift 2.45 obtained with the ESO/MPI 2.2 mtelescope equipped with the multi-channel imager GROND. Follow-up observationsin g'r'i'z' and JHK_S bands started ~17 min and extended up to 28 h post burst.We find an increase in brightness of the afterglow at early times, which can beexplained by the superposition of reverse and forward shock (FS) emission orthe onset of the afterglow FS. Additionally, we detect a strong broad-bandabsorption feature in the i' band, which we interpret as extinction from theredshifted 2175 A bump in the GRB host galaxy. This is one of the first andclearest detections of the 2175 A feature at high redshift. It is strongevidence for a carbon rich environment, indicating that Milky Way or LargeMagellanic Cloud like dust was already formed in substantial amounts in agalaxy at z=2.45.

The 2175 A dust feature in a Gamma Ray Burst afterglow at redshift 2.45