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(Abridged) Our aim is to determine 18O/17O abundance ratios across the entireGalaxy. These provide a measure of the amount of enrichment by high-mass versusintermediate-mass stars. Such ratios, derived from the C18O and C17O J=1-0lines alone, may be affected by systematic errors. Therefore, the C18O and C17O(1-0), (2-1), and (3-2), as well as the 13CO (1-0) and (2-1) lines, wereobserved towards 18 prominent galactic targets (a total of 25 positions). Thecombined dataset was analysed with an LVG model, accounting for optical deptheffects. The data cover galactocentric radii R between 0.1 and 16.9 kpc (solarcircle at 8.5 kpc). Near the centre of the Galaxy, 18O/17O = 2.88 +/- 0.11. Forthe galactic disc out to an R of ca. 10 kpc, 18O/17O = 4.16 +/- 0.09. At ca. R= 16.5 kpc, 18O/17O = 5.03 +/- 0.46. Assuming that 18O is synthesisedpredominantly in high-mass stars (M > 8 Msun), while C17O is mainly a productof lower-mass stars, the ratio from the inner Galaxy indicates a dominance ofCNO-hydrogen burning products that is also apparent in the C- and N-isotoperatios. The high 18O/17O value of the solar system (5.5) relative to that ofthe ambient ISM suggests contamination by nearby high-mass stars during itsformation. High values in the metal-poor environment of the outer Galaxy arenot matched by the low values observed towards the even more metal-poor LMC.Apparently, the outer Galaxy cannot be considered as an intermediateenvironment between the solar neighbourhood and the ISM of small metal-poorgalaxies. The apparent 18O/17O gradient along the galactic disc and thediscrepancy between outer disc and LMC isotope ratios may be explained bydifferent ages of the respective stellar populations.

Galactic interstellar 18O/17O ratios - a radial gradient?