Observed transitions of HC₃S. Quantum numbers are indicated at the right side of each row. The abscissa corresponds to the rest frequency. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvins. Blank channels correspond to negative features produced when folding the frequency-switched data. The Λ-doubling components e and f for each J are shown in the left and right panels, respectively. In addition, the two hyperfine components of each e and f rotational transition are resolved in our data and appear clearly detected. They correspond to F_u = J_u + 1/2 and J_u − 1/2. The red line shows the modelled spectra for HC₃S with the physical parameters given in Sect. 3.3.