  We report measurements of the 12C/13C abundance ratio in the three galactic
regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the 12CI
3P2-3P1 transition and the hyperfine components of the corresponding 13CI
transition near 809 GHz. These transitions were observed simultaneously with
the CO 7-6 line emission at 806 GHz with the AST/RO telescope located at the
South Pole. From a simultaneous fit to the 12CI 3P2-3P1 transition and the HF
components of the corresponding 13CI transition and an independent estimate of
an upper limit to the optical depth of the 12CI emission we determine intrinsic
12CI/13CI column density ratios of 23+-1 for G 333.0-0.4, 56+-14 for NGC 6334 A
and 69+-12 for G 351.6-1.3. As the regions observed are photon dominated, we
argue that the apparent enhancement in the abundance of 13C towards G 333.0-0.4
may be due to strong isotope-selective photodissociation of 13CO, outweighing
the effects of chemical isotopic fractionation as suggested by models of PDRs.
Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar
to the situation for the Orion Bar region observed by Keene et al. (1998).
