The three lowest rotational energy levels of OH⁺. Except for the N = 0 level, each rotational level is split into three fine structure levels. The energy difference between levels J = 1 and J = 3 of N = 2 is not resolved in the figure. The hyperfine structure, which further splits the levels, is not indicated in the figure. The transitions corresponding to the lines detected in the present work are indicated in red (with the corresponding wavelength); the dashed line represents a line detected only in NGC 6445.