Detection of C_5_N^-^ in IRC +10216

The four series must arise from linear molecules with mass and size close to those of C_6_H and C_5_N. Three of the series have half-integer rotational quantum numbers; we assign them to the ^2^{Delta} and ^2^{Sigma}^-^ vibronic states of C_6_H in its lowest ({nu}_11_) bending mode.

The fourth series, B1389, has integer J with no evidence of fine or hyperfine structure; it has a rotational constant of 1388.860(2)MHz and a centrifugal distortion constant of 33(1)Hz; it is almost certainly the C_5_N^-^ anion.