There are troubling disagreements between model and observed parameters for M dwarf stars. These disagreements are evident in two of the most well understood open clusters in the sky; the Pleiades and Praesepe. Below about 4200K, the lower stellar sequence of both clusters diverges from theoretical isochrones (e.g. Bell et al. 2012). Using robust fitting methods, we have utilised optical to mid-infrared photometry to accurately measure the luminosity and effective temperature of a sample of M stars from these clusters, and hence determine their radii. We find that the radii are systematically inflated by at least 5 percent for a given age and luminosity when compared to the corresponding stellar model. We performed an independent check of the temperature using robust optical spectrophotometry of a sample from both clusters that spans the temperature range of the radius discrepancy. The cooler effective temperatures that result from fitting the optical/IR photometry alone match molecular and atomic features in optical spectra much more closely than the temperature provided by the isochrone. Despite this marked improvement, many molecular species still show considerable discrepancies. Although there have been claims in the literature that M dwarfs are over radius, we show that our results, and those in the literature, could also be consistent with stars that are under luminous. This finding has important implications for stellar evolution and the radii of exoplanets.

{"references": ["Stauffer J. R., et al., 2007, ApJS, 172, 663", "Bell C. P. M., Naylor T., Mayne N. J., Jeffries R. D., Littlefair S. P., 2012, MNRAS, 424, 3178", "Allard F., Homeier D., Freytag B., 2011, ASPC, 448, 91A", "Baraffe I., Chabrier G., Allard F., Hauschildt P. H., 1998, A&A, 337, 403", "Dotter A., Chaboyer B., Jevremovic \u0301 D., Kostov V., Baron E., Ferguson J. W., 2008, ApJS, 178, 89", "Smette A., et al., 2015, A&A, 576, A77", "Kausch W., et al., 2015, A&A, 576, A78", "Baraffe I., Homeier D., Allard F., Chabrier G., 2015, A&A, 577, A42", "Lawrence A., et al., 2007, MNRAS, 379, 1599", "Skrutskie M. F., et al., 2006, AJ, 131, 1163"]}