  The nearby M dwarf binary GJ65AB, also known as BL Cet and UV Cet, is a
unique benchmark for investigation of dynamo-driven activity of low-mass stars.
Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as
studies of flares, photometric variability, X-ray and radio emission. Here we
present a direct analysis of large-scale and local surface magnetic fields in
both components. Interpreting high-resolution circular polarisation spectra
(sensitive to a large-scale field geometry) we uncovered a remarkable
difference of the global stellar field topologies. Despite nearly identical
masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like
global field with an average strength of 1.3 kG while the primary has a much
weaker, more complex and non-axisymmetric 0.3 kG field. On the other hand, an
analysis of the differential Zeeman intensification (sensitive to the total
magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and
6.7 kG for GJ65A and B, respectively, although there is an evidence that the
field strength distribution in GJ65B is shifted towards a higher field strength
compared to GJ65A. Based on these complementary magnetic field diagnostic
results we suggest that the dissimilar radio and X-ray variability of GJ65A and
B is linked to their different global magnetic field topologies. However, this
difference appears to be restricted to the upper atmospheric layers but does
not encompass the bulk of the stars and has no influence on the fundamental
stellar properties.
