Context : High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions , such as prestellar cores .
Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos , where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains , and thus are remnants of a previous cold ( either gas-phase or grain surface ) chemistry .
The recent detection of DCN towards the Orion Bar , in a clump at a characteristic temperature of 70 K , has shown that high deuterium fractionation can also be detected in PDRs .
The Orion Bar clumps thus appear as a good environment for the observational study of deuterium fractionation in luke-warm gas , allowing to validate chemistry models in a different temperature range , where dominating fractionation processes are predicted to be different than in cold gas ( < 20 K ) .

Aims : We aimed at studying observationally in detail the chemistry at work in the Orion Bar PDR , to understand if DCN is produced by ice mantle evaporation , or is the result of warm gas-phase chemistry , involving the CH _ { 2 } D ^ { + } precursor ion ( which survives higher temperatures than the usual H _ { 2 } D ^ { + } precursor ) .

Methods : Using the APEX and the IRAM 30 m telescopes , we targetted selected deuterated species towards two clumps in the Orion Bar .

Results : We confirmed the detection of DCN and detected two new deuterated molecules ( DCO ^ { + } and HDCO ) towards one clump in the Orion Bar PDR .
Significant deuterium fractionations are found for HCN and H _ { 2 } CO , but a low fractionation in HCO ^ { + } .
We also give upper limits for other molecules relevant for the deuterium chemistry .

Conclusions : We argue that grain evaporation in the clumps is rather unlikely to be dominant , and we find that the observed deuterium fractionation ratios are consistent with predictions of pure gas-phase chemistry models at warm temperatures ( T \sim 50 K ) .
We show evidence that warm deuterium chemistry driven by CH _ { 2 } D ^ { + } is at work in the clumps .