Optical and NIR observations of the afterglow of GRB020813

We present optical and near-infrared (NIR) photometry of the bright afterglow of GRB020813. Our data span from 3 hours to 4 days after the GRB event. A rather sharp achromatic break is present in the light curve, 14 hours after the trigger. In the framework of jetted fireballs, this break corresponds to a jet half-opening angle of 1.9degrees +/- 0.2degrees, the smallest value ever inferred for a GRB. We discuss our results in the framework of currently available models, and find that they have problems in explaining the joint temporal and spectral properties, and in particular the slow decay before the break.