TITLE: GCN CIRCULAR NUMBER: 11269 SUBJECT: GRB100909A: NOT optical afterglow confirmation DATE: 10/09/10 05:38:48 GMT FROM: Martin Jelinek at Inst.Astrophys.Andalucia,Granada Martin Jelinek, Javier Gorosabel (IAA-CSIC) and Sergio Simon (IAC-ULL) report on behalf of a larger colaboration: "We observed the optical afterglow of GRB100909A (Mereghetti et al. GCNC11260; Siegel et al. GCNC11261; Guidorzi et al. GCNC 11265) with the 2.56m Nordic Optical Telescope equipped with ALFOSC, starting on September 10 at 4:30 UT (i.e. 19.4h after the burst). A preliminary analysis of the 600-s R-band exposure shows the proposed afterglow with magnitude R~23.2 in respect to the R2=18.54 object mentioned by Guidorzi et al. (GCNC11265). This implies a decay rate alpha~1.6, and confirms the fading nature of the object. This message may be cited."