Dust model fits to optical polarisation spectra obtained with FORS for 24 sightlines. Observations are shown as the original unbinned spectrum (grey lines) and re-binned to a spectral resolution of λ/Δλ ~ 50 (black open circles). The error bars associated with the re-binned spectra correspond to 1 σ. In each panel, best-fit dust models that adopt maximum IDG alignment efficiency are shown in ochre, while models based on RAT alignment are shown by the magenta line, with individual contributions from amorphous silicates (green line), carbon grains (brown line), and micrometre-sized dust agglomerates (black line) also indicated. Data available in the stellar polarisation catalogue by Heiles (2000) are shown with a green circle.