  From observations collected with the ESPaDOnS and NARVAL spectropolarimeters,
we report the detection of Zeeman signatures on the classical T Tauri star BP
Tau. Circular polarisation signatures in photospheric lines and in narrow
emission lines tracing magnetospheric accretion are monitored throughout most
of the rotation cycle of BP Tau at two different epochs in 2006. We observe
that rotational modulation dominates the temporal variations of both
unpolarised and circularly polarised spectral proxies tracing the photosphere
and the footpoints of accretion funnels.
  From the complete data sets at each epoch, we reconstruct the large-scale
magnetic topology and the location of accretion spots at the surface of BP Tau
using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG
dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation
axis. Accretion spots coincide with the two main magnetic poles at high
latitudes and overlap with dark photospheric spots; they cover about 2% of the
stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is
very reminiscent of magnetic topologies of fully-convective dwarfs. It suggests
that magnetic fields of fully-convective cTTSs such as BP Tau are likely not
fossil remants, but rather result from vigorous dynamo action operating within
the bulk of their convective zones.
  Preliminary modelling suggests that the magnetosphere of BP Tau extends to
distances of at least 4 R* to ensure that accretion spots are located at high
latitudes, and is not blown open close to the surface by a putative stellar
wind. It apparently succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of BP Tau.
