  We present Yebes 40m telescope observations of the three most stable C4H3N
isomers towards the cyanopolyyne peak of TMC-1. We have detected 13 transitions
from CH3C3N (A and E species), 16 lines from CH2CCHCN, and 27 lines (a-type and
b-type) from HCCCH2CN. We thus provide a robust confirmation of the detection
of HCCCH2CN and CH2CCHCN in space. We have constructed rotational diagrams for
the three species, and obtained rotational temperatures between 4-8 K and
similar column densities for the three isomers, in the range (1.5-3)e12 cm-2.
Our chemical model provides abundances of the order of the observed ones,
although it overestimates the abundance of CH3CCCN and underestimates that of
HCCCH2CN. The similarity of the observed abundances of the three isomers
suggests a common origin, most probably involving reactions of the radical CN
with the unsaturated hydrocarbons methyl acetylene and allene. Studies of
reaction kinetics at low temperature and further observations of these
molecules in different astronomical sources are needed to draw a clear picture
of the chemistry of C4H3N isomers in space.
