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Observations of up to ten carbon monoxide (CO and isotopomers) transitionsare presented to study the interstellar C18O/C17O ratio towards 21 positions inthe nearby (d~140pc) low-mass star forming cloud rho Oph. A map of the C18OJ=1-0 distribution of parts of the cloud is also shown. An average12C18O/12C17O isotopomeric ratio of 4.11 +/- 0.14, reflecting the 18O/17Oisotope ratio, is derived from Large Velocity Gradient (LVG) calculations. FromLTE column densities we derive a ratio of 4.17 +/-0.26. These calculations alsoshow that the kinetic temperature decreases from about 30 K in the cloudenvelope to about 10 K in the cloud cores. This decrease is accompanied by anincrease of the average molecular hydrogen density from 10^4 cm-3 to >10^5cm-3. Towards some lines of sight C18O optical depths reach values of orderunity.Comment: 13 pages, 9 figures; accepted for publication in A&

The interstellar C$\mathsf{^{18}}$O/C$\mathsf{^{17}}$O ratio in the solar neighbourhood: The ρ Ophiuchus cloud