  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 C6H and C5N. Three of the series have half-integer rotational
quantum numbers; we assign them to the 2Delta and 2Sigma vibronic states of C6H
in its lowest (v_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 C5N- anion.

Astrophysics