TITLE: GCN GRB OBSERVATION REPORT NUMBER: 302 SUBJECT: GRB 990506 optical observations DATE: 99/05/08 21:14:49 GMT FROM: Krzysztof Z. Stanek at CfA P. M. Garnavich, K. Z. Stanek and M. R. Garcia (Harvard-Smithsonian CfA) report on behalf of the CfA GRB optical follow-up team: We measured the object at RA 11:54:57.77, Decl -26:43:02.9 (J2000) (coordinates based on the GSC catalog), reported by Vrba et al. to be variable (GCN #300), in our R-band images taken with the 1.2-meter telescope at the F. L. Whipple Obs. by N. Caldwell (May 7) and K. J. Rines (May 8). We find R=22.6 +/- 0.3 on average of two 15-min images (May 7 03:15 UT) and R=22.5 =/- 0.3 on average of two 15-min images (May 8 UT 04:48). The data were calibrated using Landolt standard star RU149. This is consistent with the Vrba et al. object being non-variable. This message is citeable.