TITLE: GCN GRB OBSERVATION REPORT NUMBER: 301 SUBJECT: GRB 990506 optical observations DATE: 99/05/08 17:33:10 GMT FROM: Paul Vreeswijk at U of Amsterdam P.M. Vreeswijk and E. Rol (University of Amsterdam) report on behalf of the Amsterdam/Huntsville GRB optical follow-up team: We measured the object at RA 11:54:57.80, Decl -26:43:03.8 (J2000) reported by Vrba et al. to be variable (GCN #300) in our Harris-R band images, taken with the WIYN telescope at May 8.18 UT. We find R = 22.21 +/- 0.15 using an aperture size which is twice the seeing disk of 1.05"). Our measurement was taken at about the same time as where Vrba et al. report Rc > about 23. The data were calibrated using the RUBIN 149 field of the Landolt catalog (Landolt et al. 1992, 104, 340). The value we find for the reference star at RA 11:54:51.6, Decl -26:43:4.09 (J2000), R = 16.1 +/- 0.1 is in good agreement with the Vrba et al. calibration for this star: R = 16.11. Based on our result we believe the object is constant, but we urge more observations to confirm/reject the variable nature of this object. This message is citeable.