TITLE: GCN GRB OBSERVATION REPORT NUMBER: 318 SUBJECT: GRB 990510 optical observations DATE: 99/05/12 03:20:47 GMT FROM: Krzysztof Z. Stanek at CfA K. Z. Stanek, P. M. Garnavich (Harvard-Smithsonian CfA), J. Kaluzny, W. Pych (Warsaw Univ. Obs.) and I. Thompson (Carnegie Obs.) report: We continued R-band observations with the 1.0-meter Swope telescope at the Las Campanas Obs. of the optical transient associated with the GRB 990510 on May 12.0 (UT). We find the OT declined by 1.4 mag since May 11.0, consistent with a power-law decay index of -1.36, considerably steeper than the index of -0.86 found from observations between May 10.7 and 11.0 (Galama et al. GCN 313; Axelrod et al. GCN 315). Compared to the star at RA 13:38:00.82 Dec -80:29:11.7 assumed to be R=16.5 (USNO A2.0 catalog), we find the OT to be R=20.3 on May 12.0 (UT). An updated light curve is available at http://cfa-www.harvard.edu/~peterg/grb.html. This message is citeable.