OH⁺ column density (shown here for HD 80558) derived from the slope of the line through points, plotting the equivalent width values as function of the transition wavelength and the oscillator strength, using Eq. (2). The line through the points is a linear fit weighted to the uncertainty of W_λ which yields a slope of (8.3 ± 0.4) × 10¹³ cm⁻². Data points are marked according to the corresponding label for the OH⁺ line (Table 3; see Fig. C.1 for the plots of the other targets.)