Using a novel technique , which combines previously determined rotation periods with new spectroscopic determinations of projected rotation velocity , we have determined radii for fast rotating , low-mass ( 0.2–0.7 M _ { \odot } ) M-dwarfs in the young , solar-metallicity open cluster , NGC 2516 .
The mean radii are larger than model predictions at a given absolute I or K magnitude and also larger than the measured radii of magnetically inactive M-dwarfs ; the difference increases from a few per cent , to 50 per cent for the lowest luminosity stars in our sample .
We show that a simple two-temperature starspot model is broadly capable of explaining these observations , but requires spot coverage fractions of about 50 per cent in rapidly rotating M-dwarfs .