Context : Low-mass prestellar cores are rarely found to be fragmented into smaller condensations but studying such substructure where present is essential for understanding the origin of multiple stellar systems .

Aims : We attempt to better understand the kinematics and dynamics of the subfragments inside the prestellar core SMM 6 in Orion B9 .
Another object of the present study is to constrain the evolutionary stage of the condensations by investigating the levels of CO depletion and deuterium fractionation .

Methods : We used the APEX telescope to observe the molecular lines C ^ { 17 } O ( 2 - 1 ) , N _ { 2 } H ^ { + } ( 3 - 2 ) , and N _ { 2 } D ^ { + } ( 3 - 2 ) towards the condensations .
We use the line data in conjunction with our previous SABOCA 350- \mu m dust continuum map of the source .

Results : The condensations are characterised by subsonic internal non-thermal motions ( \sigma _ { NT } \simeq 0.5 c _ { s } ) , and most of them appear to be gravitationally bound .
The dispersion of the N _ { 2 } H ^ { + } velocity centroids among the condensations is very low ( 0.02 km s ^ { -1 } ) .
The CO depletion factors we derive , f _ { D } = 0.8 \pm 0.4 - 3.6 \pm 1.5 , do not suggest any significant CO freeze-out but this may be due to the canonical CO abundance we adopt .
The fractional abundances of N _ { 2 } H ^ { + } and N _ { 2 } D ^ { + } with respect to H _ { 2 } are found to be \sim 0.9 - 2.3 \times 10 ^ { -9 } and \sim 4.9 - 9.9 \times 10 ^ { -10 } , respectively .
The deuterium fractionation of N _ { 2 } H ^ { + } lies in the range 0.30 \pm 0.07 - 0.43 \pm 0.09 .

Conclusions : The detected substructure inside SMM 6 is likely the result of cylindrical Jeans-type gravitational fragmentation .
We estimate the timescale for this fragmentation to be \sim 1.8 \times 10 ^ { 5 } yr .
The condensations are unlikely to be able to interact with one another and coalesce before local gravitational collapse ensues .
Moreover , significant mass growth of the condensations via competitive-like accretion from the parent core seems unfeasible .
The high level of molecular deuteration in the condensations suggests that gas-phase CO should be strongly depleted .
It also points towards an advanced stage of chemical evolution .
The subfragments of SMM 6 might therefore be near the onset of gravitational collapse , but whether they can form protostellar or substellar objects ( brown dwarfs ) depends on the local star formation efficiency and remains to be clarified .