Trends in SO peak radius (top) and fractional abundance (bottom) against the circumstellar density measure, Ṁ/υ_∞. The green lines are the trends fitted to the three higher mass-loss rate stars (R Cas, TX Cam and IK Tau), while the yellow stars are the predicted locations of the lower mass-loss rate stars (R Dor and W Hya) based on the trend. In the fractional abundance plot, the red line shows the hard limit for SO abundance based on solar S abundance and the blue points in line with the yellow stars represent the real abundance values for R Dor and W Hya.