  M star spectra, at wavelengths beyond 1.35 $\mu$m, are dominated by water
vapour yet terrestrial water vapour makes it notoriously difficult to make
accurate measurement from ground-based observations. We have used the short
wavelength spectrometer on the Infrared Space Observatory at four wavelength
settings to cover the 2.5--3.0 $\mu$m region for a range of M stars. The
observations show a good match with previous ground-based observations and with
synthetic spectra based on the Partridge & Schwenke (1997) line list though not
with the SCAN (Jorgensen et al. 2001) line list. We used a least-squared
minimisation technique to systematically find best fit parameters for the
sample of stars. The temperatures that we find indicate a relatively hot
temperature scale for M dwarfs. We consider that this could be a consequence of
problems with the Partridge & Schwenke linelist which leads to synthetic
spectra predicting water bands which are too strong for a given temperature.
Such problems need to be solved in the next generation of water vapour line
lists which will extend the calculation of water vapour to higher energy levels
with the good convergence necessary for reliable modelling of hot water vapour.
Then water bands can assume their natural role as the primary tool for the
spectroscopic analysis of M stars.
