  We report time-resolved, high-resolution optical spectropolarimetric
observations of the young double-lined spectroscopic binary V1878 Ori. Our
observations were collected with the ESPaDOnS spectropolarimeter at the
Canada-France-Hawaii Telescope through the BinaMIcS large programme. V1878 Ori
A and B are partially convective intermediate mass weak-line T Tauri stars on
an eccentric and asynchronous orbit. We also acquired X-ray observations at
periastron and outside periastron.
  Using the least-squares deconvolution technique (LSD) to combine information
from many spectral lines, we clearly detected circular polarization signals in
both components throughout the orbit. We refined the orbital solution for the
system and obtained disentangled spectra for the primary and secondary
components. The disentangled spectra were then employed to determine
atmospheric parameters of the two components using spectrum synthesis.
  Applying our Zeeman Doppler imaging code to composite Stokes $IV$ LSD
profiles, we reconstructed brightness maps and the global magnetic field
topologies of the two components. We find that V1878 Ori A and B have
strikingly different global magnetic field topologies and mean field strengths.
The global magnetic field of the primary is predominantly poloidal and
non-axisymmetric (with a mean field strength of 180 G). while the secondary has
a mostly toroidal and axisymmetric global field (mean strength of 310 G). These
findings confirm that stars with very similar parameters can exhibit radically
different global magnetic field characteristics. The analysis of the X-ray data
shows no sign of enhanced activity at periastron, suggesting the lack of strong
magnetospheric interaction at this epoch.
