Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5-m telescope , to determine projected radii for fast-rotating , low-mass ( 0.15 \leq M / M _ { \odot } \leq 0.6 ) members of the Praesepe cluster .
A maximum likelihood analysis that accounts for observational uncertainties , binarity and censored data , yields marginal evidence for radius inflation – the average radius of these stars is 6 \pm 4 per cent larger at a given luminosity than predicted by commonly-used evolutionary models .
This over-radius is smaller ( at 2-sigma confidence ) than was found for similar stars in the younger Pleiades using a similar analysis ; any decline appears due to changes occurring in higher mass ( > 0.25 M _ { \odot } ) stars .
Models incorporating magnetic inhibition of convection predict an over-radius , but do not reproduce this mass dependence unless super-equipartition surface magnetic fields are present at lower masses .
Models incorporating flux-blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in over-radius .
The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured .
This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples .
Our analyses assume random spin-axis orientations ; any alignment in Praesepe , as suggested by Kovacs ( 2018 ) , is strongly disfavoured by the broad distribution of projected radii .