  We report the first detection in space of the two doubly deuterated
isotopologues of methyl acetylene. The species CHD2CCH and CH2DCCD were
identified in the dense core L483 through nine and eight, respectively,
rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The
astronomical frequencies observed here were combined with laboratory
frequencies from the literature measured in the 29-47 GHz range to derive more
accurate spectroscopic parameters for the two isotopologues. We derive
beam-averaged column densities of (2.7 +/- 0.5)e12 cm-2 for CHD2CCH and (2.2
+/- 0.4)e12 cm-2 for CH2DCCD, which translate to abundance ratios
CH3CCH/CHD2CCH = 34 +/- 10 and CH3CCH/CH2DCCD = 42 +/- 13. The doubly
deuterated isotopologues of methyl acetylene are only a few times less abundant
than the singly deuterated ones, concretely around 2.4 times less abundant than
CH3CCD. The abundances of the different deuterated isotopologues with respect
to CH3CCH are reasonably accounted for by a gas-phase chemical model in which
deuteration occurs from the precursor ions C3H6D+ and C3H5D+, when the
ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to
gas-phase chemical reactions, rather than grain-surface processes, as
responsible for the formation and deuterium fractionation of CH3CCH in L483.
The abundance ratios CH2DCCH/CH3CCD = 3.0 +/- 0.9 and CHD2CCH/CH2DCCD = 1.25
+/- 0.37 observed in L483 are consistent with the statistically expected values
of three and one, respectively, with the slight overabundance of CHD2CCH
compared to CH2DCCD being well explained by the chemical model.
