Rotation periods obtained with the Kepler satellite have been combined with precise measurements of projected rotation velocity from the WIYN 3.5-m telescope to determine the distribution of projected radii for several hundred low-mass ( 0.1 \leq M / M _ { \odot } \leq 0.8 ) , fast-rotating members of the Pleiades cluster .
A maximum likelihood modelling technique , that takes account of observational uncertainties , selection effects and censored data , and considers the effects of differential rotation and unresolved binarity , has been used to find that the average radius of these stars is 14 \pm 2 per cent larger at a given luminosity than predicted by the evolutionary models of Dotter et al .
( 2008 ) and Baraffe et al .
( 2015 ) .
The same models are a reasonable match to the interferometric radii of older , magnetically inactive field M-dwarfs , suggesting that the over-radius may be associated with the young , magnetically active nature of the Pleiades objects .
No evidence is found for any change in this over-radius above and below the boundary marking the transition to full convection .
Published evolutionary models that incorporate either the effects of magnetic inhibition of convection or the blocking of flux by dark starspots do not individually explain the radius inflation , but a combination of the two effects might .
The distribution of projected radii is consistent with the adopted hypothesis of a random spatial orientation of spin axes ; strong alignments of the spin vectors into cones with an opening semi-angle < 30 ^ { \circ } can be ruled out .
Any plausible but weaker alignment would increase the inferred over-radius .