Population diagrams for SiO for the three stars, IRAS 15194–5115 (left), IRAS 15082–4808 (centre), and IRAS 07454–7112 (right). The solid line (red) is the best-fit linear model to the data (black). Rotation temperatures, T_rot, and source-averaged column densities, N_tot, are listed in the legends. Circle and square points denote lines observed with ALMA and APEX, respectively. Hollow circles denote lines for which data combination (see Sect. 3.3) was performed.