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We have studied the ratio of 13 CO and C 18 O column densities to the extinction A J 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 12 CO($J=1-0$) line and in $J=1-0$ and $J=2-1$ transitions of 13 CO and C 18 O using the 15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of the clouds, L 466, was also mapped in the C 18 O($J=1-0$) line. In addition to the N ( 13 CO)/ A J and N (C 18 O)/ A J ratios also the ratios of N ( 13 CO) and N (C 18 O) to A V and N (H 2 ) 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 H 2 . The alternative explanation in terms of the N (H 2 )/ A J 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)/ A J variations on the basis of previous observations in star forming and non-star forming dark clouds.

The relationship of CO abundance to extinction andN(H$\mathsf{_{2}}$):