Herschel/HIFI measurements of the ortho/para ratio in water towards
  Sagittarius B2(M) and W31C
  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.
