Fundamental transitions of HDCCC and H₂CCC in TMC-1. Also present in the upper panel is the hyperfine-split transition of C₆H in the lower Λ component (e) at 20792.872 and 20792.945 MHz. Partially resolved kinematic structure is seen in the lines of HDCCC and C₆H, but not in H₂CCC owing to the lower S/N. The dashed line at + 5.8 km s^-1 indicates the systemic velocity of TMC-1. The integration time was approximately 11.5 h for the HDCCC and 12 min for the H₂CCC spectrum. Both spectra were Hanning-smoothed to a frequency resolution of 6.1 kHz. A similar spectrum was observed by Cernicharo et al. (1987), see Fig. 2 lower left panel in their paper, and Fossé et al. (2001), see Fig. 1 upper panel. In the paper by Cernicharo et al. (1987), the line of H₂CCC was reported as an unidentified feature because the laboratory spectrum of H₂CCC was not known yet.