Observed lines of HCC³⁴S⁺ towards TMC-1. The abscissa corresponds to the rest frequency. The ordinate corresponds to the antenna temperature corrected for atmospheric and telescope losses in mK. The derived line parameters of the observed lines are given in Table B.1. The modelled spectra (red line) are derived for N(HCC³⁴S⁺)=4.7×10¹⁰ cm⁻² adopting the kinetic temperatures calculated for CC³⁴S as the rotational temperature for n(H₂)=1.3×10⁴ cm⁻³.