  We report the detection of D2CO in a sample of starless dense cores, in which
we previously measured the degree of CO depletion. The deuterium fractionation
is found extremely high, [D2CO]/[H2CO] ~ 1-10 %, similar to that reported in
low-mass protostars. This provides convincing evidence that D2CO 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 [D2CO]/[H2CO] ratios, supporting the theoretical prediction that
deuteration increases with increasing CO depletion.
