Stellar radius versus luminosity for stars in the open clusters NGC 2516, NGC 2547, NGC 2264. Crosses in the upper plots show the R sin i of individual targets normalised by the average value of sini (see Sect. 3.2). Squares with error bars show the averaged radii in bins of luminosity; adjacent numbers indicating the numbers of targets per bin. Diamonds with error bars show averaged radii (calculated in a similar way) from an alternate data set in NGC 2516 from Jackson et al. (2009). Lines show solar metallicity model isochrones of R versus log L interpolated to the cluster ages in Table 2 from several evolutionary models (see Sect. 4.1): the black solid line (with circles indicating fiducial mass points) – BHAC15 (Baraffe et al. 2015), blue dotted line – Dartmouth (Dotter et al. 2008), blue dashed – Dartmouth modified for magnetic fields (Feiden & Chaboyer 2013; Feiden et al. 2015), red dot-dashed – YREC, and red solid line – YREC modified for an effective spot coverage of 30 per cent (Somers & Pinsonneault 2015). The shaded area indicates the range of luminosities over which stars develop radiative cores according to the BHAC15 model. Stars to the left of this region have radiative cores, while stars to the right are fully convective. The lower plots show the “over-radius”, expressed as a percentage of the predicted radius from the BHAC15 model. Dashed and solid lines show the over-radius, with respect to their non-magnetic counterparts, predicted by the Dartmouth-magnetic model and the YREC with starspots model respectively.