Lines of HCCNCH⁺ observed toward TMC-1. The J = 4–3 and J = 5–4 lines were observed with the Yebes 40 m telescope and the J = 9–8 and J = 10–9 lines with the IRAM 30 m telescope. Unidentified lines are labeled as “U”. The red lines correspond to the line profiles calculated with the LVG method adopting a column density for HCCNCH⁺ of 3.0 × 10¹⁰ cm⁻², a kinetic temperature of 10 K, a volume density of H₂ of 4 × 10⁴ cm³, a linewidth of 0.60 km s⁻¹, and an emission size of 40″ in radius (see text).