  We present the discovery in TMC-1 of allenyl acetylene, H2CCCHCCH, through
the observation of nineteen lines with a signal-to-noise ratio ~4-15. For this
species, we derived a rotational temperature of 7 +/- 1 K and a column density
of (1.2 +/- 0.2)e13 cm-2. The other well known isomer of this molecule, methyl
diacetylene (CH3C4H), has also been observed and we derived a similar
rotational temperature, Trot = 7.0 +/- 0.3 K, and a column density for its two
states (A and E) of (6.5 +/- 0.3)e12 cm-2. Hence, allenyl acetylene and methyl
diacetylene have a similar abundance. Remarkably, their abundances are close to
that of vinyl acetylene (CH2CHCCH). We also searched for the other isomer of
C5H4, HCCCH2CCH (1.4-pentadiyne), but only a 3sigma upper limit of 2.5e12 cm-2
to the column density can be established. These results have been compared to
state-of-the-art chemical models for TMC-1, indicating the important role of
these hydrocarbons in its chemistry. The rotational parameters of allenyl
acetylene have been improved by fitting the existing laboratory data together
with the frequencies of the transitions observed in TMC-1.
