  We present a study of the isocyano isomers of the cyanopolyynes HC3N, HC5N,
and HC7N in TMC-1 and IRC+10216 carried out with the Yebes 40m radio telescope.
This study has enabled us to report the detection, for the first time in space,
of HCCCCNC in TMC-1 and to give upper limits for HC6NC in the same source. In
addition, the deuterated isotopologues of HCCNC and HNCCC were detected, along
with all 13C substitutions of HCCNC, also for the first time in space. The
abundance ratios of HC3N and HC5N, with their isomers, are very different in
TMC-1 and IRC+10216, namely, N(HC5N)/N(HC4NC) is 300 and >2100, respectively.
We discuss the chemistry of the metastable isomers of cyanopolyynes in terms of
the most likely formation pathways and by comparing observational abundance
ratios between different sources.
