Line-strength ratio of the cold H₂O J_Ka,Kc = 2_1,2−1_0,1 transition and the CO J = 15−14 transition as a function of Ṁ_g/ ν_∞,g. The points with error bars give the measured H₂O/CO line-strength ratios, color coded according to the values of ν_∞,g (left panel) or according to the variability type (right panel). A black cross superimposed on the data point indicates that the H₂O line is flagged as a blend. The black curves show the same predicted H₂O/CO line-strength ratios as Fig. 7 with n_H2O/n_H2 as indicated in the figures, ν_∞,g = 10 km s^-1 and ψ = 1 × 10^-3. In the left panel, the green curves show the predicted H₂O/CO line-strength ratios for ν_∞,g = 25 km s^-1. In the right panel, predictions for ψ = 0.5 × 10^-3 in red and ψ = 2 × 10^-3 in green are shown. Adopted values for other parameters are listed in Table 4.