Observed (blue) and model (red) ¹³CO lines towards R Scl (A = APEX, I = IRAM, S = SEST). The 2×10³ cm^-3/100 K and f_CO = 10^-3 model together with a ¹²CO/¹³CO ratio of 13 is used in calculating the model spectra. The velocity scale is centred on the systemic velocity estimated to be –18.9 km s^-1. The dashed lines indicate the regions used in the fitting to the line intensities.