in observing such transient sources, challenges theoretical models to explain them, the fireball model providing the best overall agreement [e.g., @Rees92; @Meszaros93; @Piran99]. In this scenario, the GRB afterglow originates from the synchrotron radiation produced by the interaction between the ultra-relativistic ejecta (jet) and the surrounding interstellar medium.\
Although GRBs can be extremely variable in their prompt phase and X-ray afterglow flares are commonly observed during the first minutes after the burst, the late-time afterglow in general shows a fairly smooth power-law behaviour at different phases [@Zhang06], from the X-rays to the optical, IR and radio wavelengths. Environmental effects and intrinsic discontinuities can introduce afterglow variability on different time scales, possibly due to *(i)* ambient density fluctuations [@Wang00], *(ii)* substructures in the jets [patchy-jet; @Meszaros98], *(iii)* inhomogeneities on the emitting surface [patchy-shell; @Kumar00], *(iv)* refreshed shocks [@Rees98; @Sari00] or *(v)* late-time central engine activity [@Rees00].\
*(i)* Density fluctuations can arise from interstellar turbulence or can be generated, before the GRB event, by a variable wind from the progenitor star [see e.g., @VanMarle05]. Linear density fluctuations with $dn/n<1$ on a length scale of 1–$10^3$ AU could induce fluctuations in the afterglow light curve with a fractional amplitude of up to $\sim30$ 