TITLE: GCN CIRCULAR NUMBER: 12637 SUBJECT: GRB 111209A: TAROT La Silla observatory optical light curve analysis DATE: 11/12/09 13:59:58 GMT FROM: Alain Klotz at CESR-CNRS Klotz A. (IRAP-OMP), Gendre B. (ASDC), Boer M. (OHP-OAMP), Atteia J.L. (IRAP-OMP) report: We obtained an optical follow-up until 4500 seconds after the trigger of GRB 111209A detected by SWIFT (trigger 509336-7) using the TAROT robotic telescope (D=25cm) located at the European Southern Observatory, La Silla observatory, Chile. In this GCN circular we describe the behaviour of the optical transcient discovered by Hoversten et al. (GCNC 12632). From 470s to 2000s, we observe a fluctuating optical emission at a mean level R~17.5 that reaches sometimes R=16.6. From 2100s to 2550s we recorded an optical flash that reached R=15.4. This peak is followed by a plateau R=16.3 between 2700s to 3700s. After 3700s the flux seems begin to decrease but the low elevation 5 degrees above horizon does not allow to conclude definitively about the phase decay. All the magnitudes are based on the assumption that the color index of the GRB in optical wavelengths is the same as the reference star NOMAD1 0431-0011481 (V-R=0.34). This message may be cited.