  Observations of up to ten carbon monoxide (CO and isotopomers) transitions
are presented to study the interstellar C18O/C17O ratio towards 21 positions in
the nearby (d~140pc) low-mass star forming cloud rho Oph. A map of the C18O
J=1-0 distribution of parts of the cloud is also shown. An average
12C18O/12C17O isotopomeric ratio of 4.11 +/- 0.14, reflecting the 18O/17O
isotope ratio, is derived from Large Velocity Gradient (LVG) calculations. From
LTE column densities we derive a ratio of 4.17 +/-0.26. These calculations also
show that the kinetic temperature decreases from about 30 K in the cloud
envelope to about 10 K in the cloud cores. This decrease is accompanied by an
increase of the average molecular hydrogen density from 10^4 cm-3 to >10^5
cm-3. Towards some lines of sight C18O optical depths reach values of order
unity.
