We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H _ { 2 } ^ { 16 } O and H _ { 2 } ^ { 18 } O in absorption towards Sagittarius B2 ( M ) and W31C .
The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3 , within the observational uncertainties .
However , somewhat unexpectedly , we derive a low ortho/para ratio of 2.35 \pm 0.35 , corresponding to a spin temperature of \sim 27 K , towards Sagittarius B2 ( M ) at velocities of the expanding molecular ring .
Water molecules in this region appear to have formed with , or relaxed to , an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust .