First Results from the CHARA Array. IV. The Interferometric Radii of   Low-Mass Stars
  We have measured the angular diameters of six M dwarfs with the CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Spectral types range from M1.0 V to M3.0 V and linear radii from 0.38 to 0.69 Rsun. These results are consistent with the seven other M-dwarf radii measurements from optical interferometry and with those for sixteen stars in eclipsing binary systems. We compare all directly measured M dwarf radii to model predictions and find that current models underestimate the true stellar radii by up to 15-20%. The differences are small among the metal-poor stars but become significantly larger with increasing metallicity. This suggests that theoretical models for low mass stars may be missing some opacity source that alters the computed stellar radii. 
D. H. Berger, D. R. Gies, H. A. McAlister, T. A. ten Brummelaar, T. J.   Henry, J. Sturmann, L. Sturmann, N. H. Turner, S. T...