loss correction of the 300V spectra. In particular, the same SED analysis, but for the optical spectrum that has not been corrected for slit losses, provides a 4 per cent change in the optical slope and 68 per cent in the $E(B-V)$, for the best fit model.\

Afterglow evolution {#sec3p4}
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Time since GRB (hr) & Instrument &\

0.61 & VLT/FORS2 & 18.74$\pm$0.05\
1.69 & VLT/FORS2 & 19.57$\pm$0.05\
55.70 & Keck-I/LRIS$^a$ & 23.97$\pm$0.07\

Figure \[fig7\] shows the afterglow time evolution in the X-ray and optical bands. We converted the X-ray light curve into monochromatic flux at 1.73 keV, the logarithmic average of the XRT band, assuming a spectral slope $\beta_\mathrm{X}=1.59$ as derived from the SED (see Section \[sec3p3\]). Early ($<12$ min) X-ray data were excluded from the fit to avoid the influence of flares. The last XRT data points were also excluded in order to avoid the contribution from a possible break at late time that cannot be constrained. We fit the X-ray light curve with a single power law with temporal slope $\alpha_\mathrm{X}=1.24\pm0.07$ (reduced $\chi^2_\nu=3.67$ for 23 dof), noting that a broken power law does not improve the fit. The high $\chi^2_\nu$ could be 