TITLE: GCN CIRCULAR NUMBER: 11170 SUBJECT: GRB 100901A - NOT optical observation DATE: 10/09/02 03:09:25 GMT FROM: Annalisa De Cia at U of Reykjavik,Science Inst. A. De Cia, P.M. Vreeswijk and P. Jakobsson (University of Iceland) report: We observed the field of GRB 100901A (Immler et al., GCN 11159) with the NOT equipped with ALFOSC in good weather conditions. Observations were carried out in the R filter for 15 minutes, with mean time 12.13 hours after the GRB trigger. We measure R=17.3 for the afterglow, not corrected for Galactic extinction, calibrated against the USNO-B1.0 1127-0027231 reference star (R=15.37) used by Andreev et al., GCN 11166 and GCN 11168. time after trigger exposure (s) R mag 12.13 hours 3 x 300 17.3 We acknowledge the careful assistence of the NOT staff, in particular Tapio Pursimo.