Evolution of the polarisation degree P of GRB 080928, both in imaging polarimetry and spectro-polarimetry mode. The afterglow is unpolarised at early times (~ 14−15 h after the trigger), while P significantly increases about one day after (at t − t₀ ~ 40.7 h). As we could only derive upper limits on the first night, we cannot properly analyse the evolution of the position angle with time.