A Comparison of the Infrared Spectra of Two Very Late-type M Dwarfs with Different Gravities

We present observations obtained during the commissioning of the upgraded Cooled Grating Spectrometer 4 on the UK Infrared Telescope on Mauna Kea, of the very low-mass objects TVLM 513-46546 and Gl 569B. Spectra were taken from 1.0 to 2.5um allowing the peak of their energy distributions to be measured. TVLM 513-46546 and Gl 569B have very similar spectral types and colours but they differ by more than a magnitude in luminosity; this indicates that their surface gravities differ by around 0.5 dex. We interpret their stellar parameters by making comparisons with the latest atmospheric models.