  To derive carbon isotopic ratios from optically thin tracers in the central
regions of the starburst galaxies M82 and NGC253. We present high sensitivity
observations of CCH and two of its 13C isotopologues, C13CH and 13CCH, as well
as the optically thin emission from C18O and 13C18O. We assume the column
density ratio between isotopologues is representative of the 12C13C isotopic
ratio. From CCH, lower limits to the 12C/13C isotopic ratio of 138 in M82, and
81 in NGC253, are derived. Lower limits to the 12C/13C ratios from CO
isotopologues support these. 13C18O is tentatively detected in NGC253, which is
the first reported detection in the extragalactic ISM. Based on these limits,
we infer ratios of 16O/18O>350 and >300 in M82 and NGC253, respectively, and
32S/34S>16 in NGC253. and the H2 column density determination through the
optically thin tracers 13CO and C18O. The derived CCH fractional abundances
toward these galaxies of <~1.1\times10^-8 are in good agreement with those of
molecular clouds in the Galactic disk. Our lower limits to the 12C/13C ratio
from CCH are a factor of 2-3 larger than previous limits. The results are
discussed in the context of molecular and nucleo-chemical evolution. The large
12C/13C isotopic ratio of the molecular ISM in these starburst galaxies suggest
that the gas has been recently accreted toward their nuclear regions.
