Spectra of sub-millimetre HCN laser transitions towards CIT 6 (left), Y CVn (middle), and S Cep (right). The Herschel/HIFI spectra are plotted in red, while the SOFIA/4GREAT spectra, smoothed to a channel spacing of ∼0.3 km s⁻¹ (except for the tentative detection the 968 GHz emission towards S Cep, smoothed to a channel spacing of 0.6 km s⁻¹), are plotted in black. The observing dates are labelled in the perspective panels. The vertical dashed blue line indicates the stellar velocity, the vertical blue bars mark the terminal velocity (listed in Table 1), the horizontal dotted blue line represents the baseline, and the grey-shaded regions indicate the velocity ranges used to determine the integrated intensities.