TITLE: GCN CIRCULAR NUMBER: 10258 SUBJECT: GRB 091208B: MITSuME Okayama Optical Observation DATE: 09/12/08 12:06:40 GMT FROM: Michitoshi Yoshida at Okayama Astrophysical Obs M. Yoshida, D. Kuroda, K. Yanagisawa, Y. Shimizu, S. Nagayama, H. Toda (OAO, NAOJ) and N. Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the field of GRB 091208B (Pagani et al., GCN 10256) with the optical three color (g', Rc and Ic) CCD camera attached to the MITSuME 50cm telescope of Okayama Astrophysical Observatory. The observation started at 09:57:03 2009-12-08 UT, 426s after the trigger. We detected the optical afterglow previously reported by de Ugarte Postigo et al. (GCN 10255) and Pagani et al. (GCN 10256) in all the three bands. The preliminary photometry results of the OT are listed below. We used GSC2.3 catalog for flux calibration. #PDAY MID-UT T-EXP g' Rc Ic ------------------------------------------------------------------ 0.00855 10:02:17 540.0 18.4 ± 0.2 17.6 ± 0.1 17.0 ± 0.2 0.01615 10:13:14 540.0 18.5 ± 0.2 18.1 ± 0.2 17.4 ± 0.2 ------------------------------------------------------------------