  It is thought that dicyanopolyynes could be potentially abundant interstellar
molecules, although their lack of dipole moment makes it impossible to detect
them through radioastronomical techniques. Recently, the simplest member of
this chemical family, cyanogen (NCCN), was indirectly probed for the first time
in interstellar space through the detection of its protonated form toward the
dense clouds L483 and TMC-1. Here we present a second firm evidence of the
presence of NCCN in interstellar space, namely the detection of the metastable
and polar isomer isocyanogen (CNCN). This species has been identified in L483
and tentatively in TMC-1 by observing various rotational transitions in the 3
mm band with the IRAM 30m telescope. We derive beam-averaged column densities
for CNCN of 1.6e12 cm-2 in L483 and 9e11 cm-2 in TMC-1, which imply fractional
abundances relative to H2 in the range (5-9)e-11. While the presence of NCCN in
interstellar clouds seems out of doubt owing to the detection of NCCNH+ and
CNCN, putting tight constraints on its abundance is still hampered by the poor
knowledge of the chemistry that links NCCN with NCCNH+ and especially with
CNCN. We estimate that NCCN could be fairly abundant, in the range 1e-9 - 1e-7
relative to H2, as other abundant nitriles like HCN and HC3N.
