G. Pietrzynski and A. Udalski (Warsaw University Observatory) report on
behalf of the OGLE microlensing survey team:

We monitored the optical counterpart of GRB 990510 (Vreeswijk et al.
1999, GCN 310) starting on May 10.992 UT, 1999 with the 1.3-m Warsaw
telescope at the Las Campanas Observatory (Chile). In total 17 V-band,
21 I-band frames were collected, covering about 10.25 hours. Exposure
time was 900 sec and 600 sec for the V and I-band frames, respectively.
The object is apparently variable -- its brightness faded by about 0.6
mag in the course of the night: from 19.45 to 20.05 in the V band and
18.50 to 19.15 in the I-band. Photometry of the optical counterpart of
GRB 990510 and VI light curves are available from the OGLE Internet
archive: http://www.astrouw.edu.pl/~ftp/ogle