TITLE: GCN CIRCULAR NUMBER: 10275 SUBJECT: GRB 091208B: GAO 150cm telescope Optical Observation DATE: 09/12/10 08:51:42 GMT FROM: Kenzo Kinugasa at Gunma Astro. Obs/Japan K. Kinugasa, S. Honda, H. Takahashi, H. Taguchi, O. Hashimoto (Gunma Astronomical Observatory) report: The position of GRB 091208B (Pagani et al., GCN 10256) was observed with the 150 cm telescope of Gunma Astronomical Observatory. Starting at 10:21:09 UT on Dec.8 ( 0.52 hours after the trigger), both Rc and Ic frames were acquired for sets of 3 x 2-min and 5 x 5-min exposures. We detected the optical counterpart (e.g., de Ugarte Postigo et al., GCN 10255; Yoshida et al. GCN 10258) in all frames. We estimated the Rc and Ic magnitudes relative to USNO-B1.0 R2 and I magnitudes, respectively. mid-UT T0+(d) exp mag. ----------------------------------------------------- 10:50:45 0.01796 3x2min, 5x3min Rc=18.8+-0.1 11:07:20 0.02273 3x2min, 5x3min Ic=18.4+-0.1 ----------------------------------------------------- The R magnitude is consistent with the fading trend of a power-law slope of ~0.7 from the other optical observations (e.g. Cano et al. GCN 10262; Xu et al. GCN 10269; Nakajima et al. GCN 10260).