Comparison of the mass fraction of hydrogen that is atomic in the CSEs of AGB stars as a function of stellar effective temperature. The y-axis shows M_FIR/M_HI, the ratio of the total (molecular plus atomic) hydrogen mass of the CSE (as estimated from FIR measurements; see Sect. 4) to the H I mass of the CSE as measured from H I 21 cm line observations. The x-axis shows T_eff, in K. Comparisons are shown as a function of chemical composition of the star (C- or O rich; top panel), and variability class (Mira, SR or L type; lower panel). Clear H I detections (det.) are shown as dots, possible detections (pos.) as smaller squares, and upper limits (lim.) as open triangles. Colours indicate different types of variability: red for Miras, black for SR types, and green for L types. FIR masses are based on 60 μm IRAS data or 70 μm Herschel images. In both panels, the colours of the points where the total hydrogen masses were derived from IRAS data are of a more vivid hue than of the Herschel data, such as red vs. orange and black vs. grey. The red vertical dashed line indicates the 2500 K soft limit around which the H₂:H I mass ratio is theoretically predicted to change (see text).