readout noise and dead time of classical CCDs usually limits the time resolution of the observation itself. High-speed photometry can now be achieved thanks to the fast read-out with zero-noise of the frame-transfer electron-multiplying CCDs (EMCCDs). The ULTRASPEC camera [@Dhillon07] adopts such a CCD to amplify the signal, rendering the read-out noise negligible. In addition, the frame transfer architecture allows the EMCCD to read out a completed exposure whilst the next exposure is being obtained, virtually eliminating the dead time between exposures.\
We observed with ULTRASPEC mounted on the ESO 3.6-m telescope in La Silla, allowing 1.09 s time resolution imaging. The highest speed photometry obtained for a GRB afterglow so far is the TORTORA observations of the extremely bright “naked eye” GRB 080319B [@Greco09; @Beskin10], with a 0.3 s time bin, from 10 to 100 s after the burst trigger. However, the ULTRASPEC observations of , presented in this paper, probes the afterglow phase, providing the lowest $\Delta t / t$ so far. This opens a new window on the fast-variability study of the afterglow itself. Comparing the variability limits given by @Ioka05 with the ULTRASPEC observations, we can constrain the properties of the circumburst medium and the shock 