  The CM Draconis system comprises two eclipsing mid-M dwarfs of nearly equal
mass in a 1.27-day orbit. This well-studied eclipsing binary has often been
used for benchmark tests of stellar models, since its components are amongst
the lowest mass stars with well-measured masses and radii (~ 1% relative
precision). However, as with many other low-mass stars, non-magnetic models
have been unable to match the observed radii and effective temperatures for CM
Dra at the 5-10% level. To date, the uncertain metallicity of the system has
complicated comparison of theoretical isochrones with observations. In this
Letter, we use data from the SpeX instrument on the NASA Infrared Telescope
Facility (IRTF) to measure the metallicity of the system during primary and
secondary eclipses, as well as out of eclipse, based on an empirical
metallicity calibration in the H and K near-infrared (NIR) bands. We derive a
[Fe/H] = -0.30 +- 0.12 that is consistent across all orbital phases. The
determination of [Fe/H] for this system constrains a key dimension of parameter
space when attempting to reconcile model isochrone predictions and
observations.
