M star spectra beyond 1.35 microns are dominated by water vapour
yet terrestial water vapour makes it notoriously difficult to make
accurate measurement of the water vapour bands. We propose
observations of the strongest water vapour absorption, impossible
from terrestial sites, from 2.5--2.9 microns in a range of M
stars.  ISO provides the resolution to detect ro-vibrational
water transitions.  We will be able to compare these observations
with our predictions based on ab initio calculations of water
vapour incorporated into the most sophisticated models for
low-mass stars and brown dwarfs.  Such comparisons will show
up any deficiencies in the model treatment of water vapour
at high temperatures and pressures and be essential for the
construction of a reliable HR diagram for all O-rich objects
below 3500 K (dwarfs and giants). The sensitivity of water vapour
absorption strength to temperature, pressure and abundance is
expected to yield accurate temperatures, metallicities and
gravities for the commonest stars. The depth of the water
vapour absorption bands through this spectral region, expected
to be around 60\% for VB10 represents a significant uncertainty
in the determination of the bolometric luminosity for late-type
stars.  These observations will remove this uncertainty and
enable the bolometric luminosity of late-type stars to be
accurately determined. In additional, these observations are
expected to play a vital role towards understanding high mass
brown dwarfs for which this region will be close to the flux
maxima.