Line parameters of the RRLs detected in CRL 618, MWC 922, and M 2-9 derived from the observed profiles (Table 2; filled circles) and from the models in Table 4 including non-detections (empty circles). Dotted lines are fits to the observed line parameters of the Hnα transitions (red filled circles). For CRL 618, we show the mean V_LSR of the Hnα centroids (dotted line) and a linear fit of V_LSR as a function of frequency (dashed line).