  The nearby red dwarf binary GJ65AB (UB+BL Ceti, M5.5Ve+M6Ve) is a cornerstone
system to probe the physics of very low mass stars. We present new
interferometric measurements of the angular diameters of the two components of
GJ65AB with the PIONIER instrument in the H band: thetaUD (A) = 0.558 +/- 0.008
+/- 0.020 mas and thetaUD (B) = 0.539 +/- 0.009 +/- 0.020 mas. They translate
into limb-darkened angular diameters of LD (A) = 0.573 +/- 0.021 mas and LD (B)
= 0.554 +/- 0.022 mas. From the known parallax, the radii are R(A) = 0.165 +/-
0.006 Rsun and R(B) = 0.159 +/- 0.006 Rsun (sigma(R)/R = 4%). We also observed
GJ65AB with the VLT/NACO adaptive optics and refine the orbital parameters and
infrared magnitudes of the system. We derive masses for the two components of
m(A) = 0.1225 +/- 0.0043 Msun and m(B) = 0.1195 +/- 0.0043 Msun (sigma(m)/m =
4%). To derive the radial and rotational velocities of the two stars, as well
as their relative metallicity with respect to Proxima, we also present new
individual UVES high resolution spectra. Placing GJ65 A&B in the mass-radius
diagram shows that their radii are larger respectively by 14 +/- 4% and 12 +/-
4% compared to expectations from the models by Baraffe et al. (2015). Following
Chabrier et al. (2007), we propose that this discrepancy is caused by the
inhibition of convective energy transport by a strong internal magnetic field
generated by dynamo effect in these two fast rotating stars. A comparison with
the almost identical twin but slowly-rotating Proxima strengthens this
hypothesis, as the radius of Proxima does not appear inflated compared to
models.
