^ { 12 } C / ^ { 13 } C line intensity ratios have been derived from several carbon-bearing molecules to confine the range of carbon and oxygen isotope abundance ratios toward the nuclear regions of two infrared bright galaxies .
The most stringent limits are obtained from CN mm-wave emission lines .
Supplementary measurements toward the Galactic center region indicate that overall I ( ^ { 12 } CN ) / I ( ^ { 13 } CN ) line intensity ratios are giving lower limits to the corresponding ^ { 12 } C / ^ { 13 } C abundance ratio .
Toward M 82 and IC 342 , we find ^ { 12 } C / ^ { 13 } C > 40 and > 30 .
Therefore the smaller ^ { 12 } C / ^ { 13 } C ratio of our own Galactic center region ( 25 ) may not be typical for central regions of galaxies which are more luminous in the infrared .
The ^ { 12 } C / ^ { 13 } C limits and data from various isotopic species of CO also infer ^ { 16 } O / ^ { 18 } O abundance ratios of > 90 and > 125 for M 82 and IC 342 , respectively .
From the I ( HC ^ { 14 } N ) / I ( HC ^ { 15 } N ) line intensity ratio , ^ { 14 } N / ^ { 15 } N > 100 is derived for M 82 .