Q/I, U/I plot from the IPOL observation on the night of 2008 September 28. The position of the optical afterglow (red bars) does not differ markedly from the other points, which represent some bright field stars observed near the transient and expected to be unpolarised. Therefore, the polarisation of the OA is consistent with zero, and only a 3σ upper limit is given (see value above the plot).