Context : Despite the low elemental deuterium abundance in the Galaxy , enhanced molecular D/H ratios have been found in the environments of low-mass star forming regions , and in particular the Class 0 protostar IRAS 16293-2422 .

Aims : The CHESS ( Chemical HErschel Surveys of Star forming regions ) Key Program aims at studying the molecular complexity of the interstellar medium .
The high sensitivity and spectral resolution of the HIFI instrument provide a unique opportunity to observe the fundamental 1 _ { 1 , 1 } – 0 _ { 0 , 0 } transition of the ortho–D _ { 2 } O molecule , inaccessible from the ground , and to determine the ortho-to-para D _ { 2 } O ratio .

Methods : We have detected the fundamental transition of the ortho-D _ { 2 } O molecule at 607.35 GHz towards IRAS 16293-2422 .
The line is seen in absorption with a line opacity of 0.62 \pm 0.11 ( 1 \sigma ) .
From the previous ground-based observations of the fundamental 1 _ { 1 , 0 } –1 _ { 0 , 1 } transition of para–D _ { 2 } O seen in absorption at 316.80 GHz we estimate a line opacity of 0.26 \pm 0.05 ( 1 \sigma ) .

Results : We show that the observed absorption is caused by the cold gas in the envelope of the protostar .
Using these new observations , we estimate for the first time the ortho to para D _ { 2 } O ratio to be lower than 2.6 at a 3 \sigma level of uncertainty , to be compared with the thermal equilibrium value of 2:1 .

Conclusions :