  We report on the discovery of the C7N- anion towards the starless core TMC-1
and towards the carbon-rich evolved star IRC+10216. We used the data of the
QUIJOTE line survey towards TMC-1 and found six lines in perfect harmonic
frequency relation from J=27-26 up to J=32-31. The frequency of the lines can
be reproduced with a rotational constant and a distortion constant of
B=582.68490+/-0.00024 MHz and D=4.01+/-0.13 Hz, respectively. The standard
deviation of the fit is 4 kHz. Towards IRC+10216, we identify 17 lines from
J=27-26 up to J=43-42; their frequencies are also in harmonic relation,
providing B=582.6827+/-0.00085 MHz and D=3.31+/-0.31 Hz. The nearly exact
coincidence of the rotational and distortion constants in both sources points
unambiguously to a common molecular carrier. Taking into account the chemical
peculiarities of both sources, the carrier could be a radical or an anion. The
radical can be discarded, as the observed lines belong to a singlet species.
Hence, the most plausible carrier is an anion. High-level ab initio
calculations indicate that C7N-, for which we compute a rotational constant of
B=582.0 MHz and a dipole moment of 7.5 D, is the carrier of the lines in both
sources. We predict the neutral C7N to have a ground electronic state 2Pi and a
dipole moment around 1 D. Because of this low dipole moment value and to its
much larger rotational partition function, its lines are expected to be well
below the sensitivity of our data for both sources.
