TITLE: GCN GRB OBSERVATION REPORT NUMBER: 316 SUBJECT: GRB990510 optical observations DATE: 99/05/11 13:06:28 GMT FROM: Andrzej Udalski at Warsaw U. Observatory 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. Standard VI photometry of three, well separated nearby constant stars (uncertainty +/-0.03 mag) is: Delta X Delta Y \alpha_2000 \delta_2000 V I from GRB position Star 1 33.4"E 43.9"S 13:38:20.56 -80:30:31.9 15.617 14.636 Star 2 59.8"W 21.0"N 13:37:43.48 -80:29:26.7 16.203 15.161 Star 3 16.5"W 36.7"N 13:38:00.82 -80:29:11.5 17.014 16.094 Photometry of the optical counterpart of GRB 990510 and light curves are available from the OGLE Internet archive: http://www.astrouw.edu.pl/~ftp/ogle [GCN OP NOTE: This circular was received at 12:42 11 May 99 UT, but was not distributed due to not having a prior vetted account.]