C¹⁸O(1−0)/C¹⁷O(1−0) intensity ratio as a function of the H₂ column density used to determine the optical depth of the rare isotopologs. The dashed horizontal line represents the standard isotopic ratio of 3.2 (Wilson & Rood 1994), and title good match with the observations indicates that both transitions are optically thin in all three clouds. Note the restricted range of the x-axis due to the low S/N of the line ratio for very low values of N(H₂).