  We report here the first results of a multi-wavelength campaign focussing on
magnetospheric accretion processes within the close binary system V4046 Sgr,
hosting two partly-convective classical T Tauri stars of masses ~0.9 Msun and
age ~12 Myr. In this paper, we present time-resolved spectropolarimetric
observations collected in 2009 September with ESPaDOnS at the
Canada-France-Hawaii Telescope (CFHT) and covering a full span of 7d or ~2.5
orbital/rotational cycles of V4046 Sgr. Small circularly polarised Zeeman
signatures are detected in the photospheric absorption lines but not in the
accretion-powered emission lines of V4046 Sgr, thereby demonstrating that both
system components host large-scale magnetic fields weaker and more complex than
those of younger, fully-convective cTTSs of only a few Myr and similar masses.
  Applying our tomographic imaging tools to the collected data set, we
reconstruct maps of the large-scale magnetic field, photospheric brightness and
accretion-powered emission at the surfaces of both stars of V4046 Sgr. We find
that these fields include significant toroidal components, and that their
poloidal components are mostly non-axisymmetric with a dipolar component of
50-100G strongly tilted with respect to the rotation axis; given the similarity
with fields of partly-convective main-sequence stars of similar masses and
rotation periods, we conclude that these fields are most likely generated by
dynamo processes. We also find that both stars in the system show cool spots
close to the pole and extended regions of low-contrast, accretion-powered
emission; it suggests that mass accretion is likely distributed rather than
confined in well defined high-contrast accretion spots, in agreement with the
derived magnetic field complexity.
