OH population diagram for R Hya, S Pav and R Aql. The number near each data point is the line number used in Table 3 of observable OH transitions. The vertical bar in each data point includes the ±1σ formal error of the integrated flux density and a rough estimate of the filling factor uncertainty. (An arrow indicate an upper limit.) Τhe green line is the regression line across the data points (see Sect. 9.2).