TITLE: GCN GRB OBSERVATION REPORT NUMBER: 498 SUBJECT: GRB 991216 Optical Observations DATE: 99/12/19 12:15:14 GMT FROM: Brian Lindgren Jensen at U.of Copenhagen B. L. Jensen, H. Pedersen, J. Hjorth, J. Pritchard (U. of Copenhagen), T. Abbott (NOT), A. J. Castro-Tirado (LAEFF-INTA, Madrid), E. Pian (ITESRE, Bologna), P. Vreeswijk (U. of Amsterdam) and J. Greiner (AIP, Potsdam) report on behalf of a larger European collaboration: "We have obtained R-band exposures of the optical counterpart (GCN #472) using the Danish 1.5m at La Silla (Dec. 18) and the 2.5m Nordic Optical Telescope on La Palma (Dec. 19). When measured relative to star "A" (GCN #476, #486) we obtain the following magnitudes: Date (UT) R Exp FWHM Dec. 18.11 : 20.12 +-0.1 3x300s 1.8" Dec. 18.32 : 20.40 +-0.1 3x300s 1.9" Dec. 19.10 : 20.90 +-0.1 4x300s 1.8" When analyzed jointly with observations available from GCNs (excluding the Rc~21.1 from GCN #487) we deduce a power-law decay index of -1.17+-0.10. The corresponding plot is available at: http://www.astro.ku.dk/~brian_j/grb/grb991216/ ."