The relationship of CO abundance to extinction andN(H₂):

We have studied the ratio of 13CO and C18O column densities to the extinction AJ of background field stars in the direction of three globules: B 133, B 335, L 466. Selected positions in the globules were observed in the 12CO() line and in and transitions of 13CO and C18O using the 15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of the clouds, L 466, was also mapped in the C18O() line. In addition to the N(13CO)/AJ and N(C18O)/AJ ratios also the ratios of N(13CO) and N(C18O) to AV and N(H2) are given. These ratios were found to vary from cloud to cloud so that they are larger in B 335 than in B 133 and L 466. These variations are thought to primarily arise from variations of the column density ratio of CO and its isotopes to H2. The alternative explanation in terms of the N(H2)/AJ ratio being larger in starforming regions (B 335) than in more quiescent regions lacking star formation (B 133 and L 466) appears less likely. We also discuss the evidence for N(CO)/AJ variations on the basis of previous observations in star forming and non-star forming dark clouds.