We report on 13 new high-precision measurements of stellar diameters for low-mass dwarfs obtained by means of near-infrared long-baseline interferometry with PIONIER at the Very Large Telescope Interferometer .
Together with accurate parallaxes from Gaia DR2 , these measurements provide precise estimates for their linear radii , effective temperatures , masses , and luminosities .
This allows us to refine the effective temperature scale , in particular towards the coolest M-dwarfs .
We measure for late-type stars with enhanced metallicity slightly inflated radii , whereas for stars with decreased metallicity we measure smaller radii .
We further show that Gaia DR2 effective temperatures for M-dwarfs are underestimated by \sim 8.2 % and give an empirical M _ { G } - T _ { eff } relation which is better suited for M-dwarfs with T _ { eff } between 2600 and 4000 K. Most importantly , we are able to observationally identify a discontinuity in the T _ { eff } -radius plane , which is likely due to the transition from partially convective M-dwarfs to the fully convective regime .
We found this transition to happen between 3200 K and 3340 K , or equivalently for stars with masses \approx 0.23 M _ { \sun } .
We find that in this transition region the stellar radii are in the range from 0.18 to 0.42 R _ { \sun } for similar stellar effective temperatures .