TITLE: GCN CIRCULAR NUMBER: 11180 SUBJECT: GRB 100901A: 1.5m OSN BVRI-band observations. DATE: 10/09/02 19:34:45 GMT FROM: Juan Carlos Tello at IAA-CSIC R. Sánchez-Ramírez, J.C. Tello, A. Sota, J. Gorosabel, A.J. Castro-Tirado (IAA-CSIC, Granada), report on behalf of a larger collaboration: "We have taken several series of BVRI-band images of the Swift GRB 100901A (Immler et al. GCNC 11159) optical afterglow (Guidorzi et al. GCNC 11160) with the 1.5m telescope at Observatorio de Sierra Nevada, Spain. The afterglow is well detected in the four bands, with preliminary average magnitudes (calibrated against USNO B1.0 and GSC 3.2 stars) of B~18.0 (Sep 2.021 UT), V~17.4 (Sep 2.035 UT), R~17.3 (Sep 2.003 UT) and I~17.3 (Sep 2.049 UT)."