Ratios of CO(1–0) and H I integrated line fluxes, I_CO,corr/F_HI in K/Jy, as a function of the ratio of FIR (H₂ + H I) to H I masses, M_FIR/M_HI. All published CO fluxes were converted to a telescope diameter of 20 m. Comparisons are shown as a function of chemical composition, C- or O-rich (top panel), and variability: 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. Data points based on H I measurements made with the VLA have been identified. Horizontal arrows indicate points whose M_FIR/M_HI mass ratios are based on Herschel FIR flux densities, which are systematically lower than IRAS values (see Sect. 4). The colours indicate the various types of objects. Top panel: O-rich (blue), or C-rich (black); lower panel: Mira (red), semi-regular variable (SR, black), and long-period variable (L, green). To guide the eye, a dashed blue line indicates a slope of 1:1 in each panel.