We report the detection in the envelope of the C-rich star IRC+10216 of four series of lines with harmonically related frequencies: B1389, B1390, B1394, and B1401. 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.