  This paper exploits spectropolarimetric data of the classical T Tauri star CI
Tau collected with ESPaDOnS at the Canada-France-Hawaii Telescope, with the
aims of detecting and characterizing the large-scale magnetic field that the
star hosts, and of investigating how the star interacts with the inner regions
of its accretion disc through this field. Our data unambiguously show that CI
Tau has a rotation period of 9.0d, and that it hosts a strong, mainly poloidal
large-scale field. Accretion at the surface of the star concentrates within a
bright high-latitude chromospheric region that spatially overlaps with a large
dark photospheric spot, in which the radial magnetic field reaches -3.7kG. With
a polar strength of -1.7kG, the dipole component of the large-scale field is
able to evacuate the central regions of the disc up to about 50% of the
co-rotation radius (at which the Keplerian orbital period equals the stellar
rotation period) throughout our observations, during which the average
accretion rate was found to be unusually high. We speculate that the magnetic
field of CI Tau is strong enough to sustain most of the time a magnetospheric
gap extending to at least 70% of the co-rotation radius, which would explain
why the rotation period of CI Tau is as long as 9d. Our results also imply that
the 9d radial velocity (RV) modulation that CI Tau exhibits is attributable to
stellar activity, and thus that the existence of the candidate close-in massive
planet CI Tau b to which these RV fluctuations were first attributed needs to
be reassessed with new evidence.
