Modelled R_FIR for different grain sizes. Top panel: observed values for R_FIR for C-rich sources (red dots) and O-rich sources (blue squares) and their uncertainty ΔR_FIR. The grey bars show the range of R_FIR that the models produce for grain sizes between 0.01 and 5.0µm. Bottom panels: examples of the model R_FIR vs. grain size for the carbon star UU Aur (left) and the O-rich star V1943 Sgr (right). The horizontal solid lines show the observed R_FIR, the dashed horizontal lines indicate the uncertainty ΔR_FIR.