We report the detection of D _ { 2 } CO in a sample of starless dense cores , in which we previously measured the degree of CO depletion .
The deuterium fractionation is found extremely high , { [ D _ { 2 } CO ] / [ H _ { 2 } CO ] } \sim 1 - 10 % , similar to that reported in low-mass protostars .
This provides convincing evidence that D _ { 2 } CO is formed in the cold pre-stellar cores , and later desorbed when the gas warms up in protostars .
We find that the cores with the highest CO depletions have also the largest [ D _ { 2 } CO ] / [ H _ { 2 } CO ] ratios , supporting the theoretical prediction that deuteration increases with increasing CO depletion .