Water vapour in cool dwarf stars

M dwarf spectra beyond 1.35 mum are dominated by water vapour yet terrestrial water vapour makes it notoriously difficult to make accurate measurement of the water vapour bands from ground-based observations. We have used the short wavelength spectrometer on the Infrared Space Observatory (ISO) at four wavelength settings to cover the 2.5-3.0 mum region for a range of M dwarfs. The observations show a good match with previous groundbased observations and a reasonable match with the best available synthetic spectra. In particular the synthetic spectra and particularly the ab initio calculations for wa ter vapour on which they hinge now accurately predict the positions of water vapour transitions. However, further work extending the calculation of water vapour to higher energy levels is probably necessary before a detailed match in intensity will be possible.