Results of GRB 080928 spectroscopic analysis. Top: GRB 080928 normalised spectrum obtained from FORS1/PMOS data taken from t ~ 14.95 h after the burst on 2008 September 28. Spectral lines from GRB 080928 absorption system at z = 1.6919 are marked with vertical solid lines, while the ones from an intervening system at z=0.7359 are represented by vertical dashed lines (for details, see Fynbo et al. 2009). Bottom: Q/I, U/I Stokes parameters spectra derived from FORS1/PMOS data. The highlighted yellow region corresponds to the V-band in which IPOL data were taken in order to perform a consistent comparison between data taken with the two observing modes. The spectra have been rebinned as described in the text.