Spectroscopic and eclipsing binary systems offer the best means for determining accurate physical properties of stars , including their masses and radii .
The data available for low-mass stars have yielded firm evidence that stellar structure models predict smaller radii and higher effective temperatures than observed , but the number of systems with detailed analyses is still small .
In this paper we present a complete reanalysis of one of such eclipsing systems , CM Dra , composed of two dM4.5 stars .
New and existing light curves as well as a radial velocity curve are modeled to measure the physical properties of both components .
The masses and radii determined for the components of CM Dra are M _ { 1 } = 0.2310 \pm 0.0009 M _ { \odot } , M _ { 2 } = 0.2141 \pm 0.0010 M _ { \odot } , R _ { 1 } = 0.2534 \pm 0.0019 R _ { \odot } , and R _ { 2 } = 0.2396 \pm 0.0015 R _ { \odot } .
With relative uncertainties well below the 1 % level , these values constitute the most accurate properties to date for fully convective stars .
This makes CM Dra a valuable benchmark for testing theoretical models .
In comparing our measurements with theory , we confirm the discrepancies reported previously for other low-mass eclipsing binaries .
These discrepancies seem likely to be due to the effects of magnetic activity .
We find that the orbit of this system is slightly eccentric , and we have made use of eclipse timings spanning three decades to infer the apsidal motion and other related properties .