Theoretical O VII Heα and β line equivalent width ratio as a function of line width b. The solid curves show the EW_Heα/EW_Heβ -ratio for various O VII ion column densities, as labeled. The dashed lines show a selection of constant EW_Heα curves in this plane, excluding the parameter regions where the associated ion column densities exceed N_OVII = 10¹⁷ cm⁻². When moving, at fixed column density (i.e., along a solid curve), from high to low b–values, the equivalent width ratio decreases gradually from its optically thin value, due to the stronger saturation of the Heα line relative to that of the weaker β line. When Heα line saturation reaches the damped regime, the opposite behaviour takes place. These properties suggest that the EW_Heα/EW_Heβ -ratio can be utilized as a sensitive diagnostic tool for b in the range of X-ray detectable O VII ion column densities, especially for low line-of-sight gas velocity dispersions.