Dense gas in nearby galaxies. VI. A large 12C/13C ratio in a nuclear starburst environnement.

Toward the starburst nucleus of NGC 253, ^12^C/^13^C line intensity ratios from six carbon bearing molecules (CO, CN, CS, HCN, HCO+, and HNC) are used to confine the possible range of carbon and oxygen isotope ratios. A detailed analysis yields ^12^C/^13^C~40 and ^16^O/^18^O~200. Also reported are first detections of ^13^CS and of the 0_0_ -1_-1_ line of methanol (CH_3_ OH) in an extragalactic source.