Observed lines of HCCCN, DCCCN, H¹³CCCN, HC¹³CCN, HCC¹³CN, and HCCC¹⁵N towards TMC-1 (CP) at 7 mm (histogram black spectra). The red curves show the synthetic spectra obtained using the LVG approximation (see Sect. 3.1 and Table A.5). The weak satellite hyperfine lines (S_w for HCCCN and S_r and S_b for the isotopologues) have been offseted and multiplied by a factor of 30. These hyperfine lines correspond to the transitions F_u − F_l = J − J for S_r, F_u − F_l = (J − 1)−J for S_w, and F_u − F_l = (J − 1)−(J − 1) for S_b.