Observed lines of CH₃CO⁺ towards TMC-1. The abscissa corresponds to rest frequencies (in MHz) assuming a local standard of rest velocity of 5.83 km s⁻¹ (Cernicharo et al. 2020a,b). Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature (in mK). Spectral resolution is 38.1 kHz below 50 GHz and 48.8 kHz above. The blue labels correspond to the series of lines we assign to the A species of CH₃CO⁺, while the red ones correspond to those of the E species.