Ratio of different CO isotopologues as a function of Galactocentric radius, as determined from the procedure described in Sect. 4.4. Each group of sources is shown with a different symbol and colour, as indicated. Velocity components different from the main one are indicated with crosses; the colour of the cross refers to the main component classification. In a) we show [¹²C]/[¹³C] for the sources detected in . The black solid line is the best fit, the yellow-shaded area in the left panel indicates the 68% uncertainty, and the dashed lines show the intrinsic scatter of the relation. In the right panel we show the joint probability distribution of the parameters of the fit (y = αx + β); in black we indicate the 68% contour. Panel b) is the same as the left panel in a), for [¹⁸O]/[¹⁷O]. Panel c) shows [¹²C]/[¹³C] for the sources undetected in (see text). We show again the fit from the left panel in a) for clarity.