1999A&A...352L..26B

We present BVRI photometry and spectrophotometry of GRB990510 obtained with the ESO VLT/Antu telescope during the late decline phase. Between days 8 and 29 after the burst, the afterglow faded from R=24.2 to ∼26.4. The spectral flux distribution and the light curve support the interpretation of the afterglow as synchrotron emission from a jet. The light curve is consistent with the optical transient alone but an underlying SN with maximum brightness R>27.4 or a galaxy with R>27.6 (3-σ upper limits) cannot be ruled out. To a 5-σ detection threshold of R=26.1, no galaxy is found within 6" of the transient. A very blue V≃24.5 extended object which may qualify as a starburst galaxy is located 12" SE, but at unknown redshift.