  We report the detection of two isomers of ethynyl cyclopentadiene
(c-C5H5CCH), namely 1- and 2-ethynyl-1,3-cyclopentadiene, in the direction of
TMC-1. We derive column densities of (1.4 +/- 0.2)e12 cm-2, and (2.0 +/-
0.4)e12 cm-2, respectively, for these two cyclopentadiene derivatives, which
imply that they are about ten times less abundant than cyclopentadiene. We also
report the tentative detection of ethynyl benzene (C6H5CCH), for which we
estimate a column density of (2.5 +/-0.4)e12 cm-2. We derived abundances for
the corresponding cyano derivatives of cyclopentadiene and benzene and found
values significantly lower than previously reported. The rotational temperature
of the ethynyl and cyano derivatives of these cycles is about 9 K, that is,
very close to the gas kinetic temperature of the cloud. The abundance ratio of
the 1- and 2- isomers of ethynyl cyclopentadiene is 1.4 +/-0.5, while for the
two isomers of cyano cyclopentadiene it is 2.4 +/- 0.6. The relative abundances
of CCH over CN derivatives is 7.7 +/- 2.2 for cyclopentadiene, which probably
reflects the abundance ratio of the radicals CCH and CN; this ratio is only 2.1
+/- 0.5 for benzene, which suggests that additional reactions besides cyano
radicals with benzene are involved in the formation of benzonitrile. The
formation of these cycles is reasonably well accounted for through a chemical
scheme based on neutral-neutral reactions. It is predicted that benzene should
be as abundant as cyclopentadiene in TMC-1.
