  We present the first astronomical detection of a diatomic negative ion, the
cyanide anion CN-, as well as quantum mechanical calculations of the excitation
of this anion through collisions with para-H2. CN- is identified through the
observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star
envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles
indicate that CN-, like the large anion C6H-, is formed in the outer regions of
the envelope. Chemical and excitation model calculations suggest that this
species forms from the reaction of large carbon anions with N atoms, rather
than from the radiative attachment of an electron to CN, as is the case for
large molecular anions. The unexpectedly large abundance derived for CN-, 0.25
% relative to CN, makes likely its detection in other astronomical sources. A
parallel search for the small anion C2H- remains so far unconclusive, despite
the previous tentative identification of the J = 1-0 rotational transition. The
abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 %
relative to C2H.
