Marginal probability map of the optical index (n, k) of the dust based on the polarised fraction and NICMOS SPF. We overplotted optical constants of amorphous carbon (Zubko et al. 1996; BE = BEnzene burning, ACAR = arc discharge between Amorphous Carbon electrodes in an ARgon atmosphere), water ice, olivine organics, iron and troilite from Pollack et al. (1994) and astrosilicates from Draine & Lee (1984). The red (respectively purple) line shows a mixture of astrosilicates (resp. amorphous carbon) with troilite.