hours after the burst can provide important constraints on the different proposed scenarios, and therefore on the physics of the evolution of the fireball.\
Temporal variations in GRB afterglow light curves were first observed, on time scale as short as $\sim1$ hour, in GRB011211 [@Holland02; @Jakobsson04], induced either by inhomogeneities in the medium surrounding the GRB, or by a patchy jet. @Lazzati03 have argued that the deviations in the afterglow of GRB021004 are due to the interaction of the GRB fireball, or jet, with density enhancements in the ambient medium. However, time-resolved polarimetry of the same burst suggested that the variations were produced by a refreshed shock [@Bjornsson04]. @Granot03 have also interpreted the variations seen in the light curve of GRB030329 as due to refreshed shocks.\
Several limitations challenge the detection of late-time variability in GRB light curves. First, the amplitude of most of the fluctuations that can possibly be expected decays with time. Moreover, different processes (e.g., density fluctuations, patchy shell or refreshed shocks) will physically constrain the variability over only certain time scales at one observation time [@Ioka05]. In particular, the fastest variability is the hardest to detect, partly because it is intrinsically weaker, but also because the 