Observed transitions of HCCCO in TMC-1. The abscissa corresponds to the rest frequency of the lines. Frequencies and intensities for the observed lines are given in Table A.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in milliKelvin. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectrum for this species for T_r = 7 K and a column density of 1.3 × 10¹¹ cm⁻². Blank channels correspond to negative features produced in the folding of the frequency switching data.