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 ( \lesssim 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 \pm 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 .