Observed (blue) and model (red) ¹²CO lines towards S Sct (A = APEX, S = SEST). The 2×10² cm^-3/100 K and f_CO = 10^-3 model is used to calculate the model spectra. The velocity scale is centred on the systemic velocity estimated to be 14.9 km s^-1. The dashed lines indicate the regions used in the fitting to the line intensities. The feature at the systemic velocity originates in the present-day wind and is not part of the modelling.