Observed spectra of the J = 2 − 1 transitions of CCH, C¹³CH, and ¹³CCH, and the J = 1−0 transitions of C¹⁸O and ¹³C¹⁸O toward M 82 and NGC 253. The intensity of the C¹⁸O spectrum in M 82 has been multiplied by a factor of 3 for comparison and visualization purposes. The fit to the CCH and C¹⁸O profiles are shown with grey lines. For the undetected isotopologues we overlay the expected spectra assuming a ¹²C/¹³C isotopic ratio of 40 (dotted lines), as well as the upper limit profile derived in this work (grey lines). Vertical lines represent the relative intensities of the hyperfine components used to fit the detected CCH profiles, and to calculate the upper limit to the intensities for the isotopologues. ¹³C¹⁸O is tentatively detected blended to a H₂CS feature toward NGC 253. Spectra are presented at the original 4 and 2 MHz (~7 and 5  kms^-1) resolution for CCH and C¹⁸O, respectively. C¹³CH and ¹³CCH data were smoothed to 16 MHz (~28 kms^-1) and ¹³C¹⁸O to 8 MHz (~23 kms^-1).