  Models of magnetospheric accretion on to classical T Tauri stars often assume
that stellar magnetic fields are simple dipoles. Recently published surface
magnetograms of BP Tau and V2129 Oph have shown, however, that their fields are
more complex. The magnetic field of V2129 Oph was found to be predominantly
octupolar. For BP Tau the magnetic energy was shared mainly between the dipole
and octupole field components, with the dipole component being almost four
times as strong as that of V2129 Oph. From the published surface maps of the
photospheric magnetic fields we extrapolate the coronal fields of both stars,
and compare the resulting field structures with that of a dipole. We consider
different models where the disc is truncated at, or well-within, the Keplerian
corotation radius. We find that although the structure of the surface magnetic
field is particularly complex for both stars, the geometry of the larger scale
field, along which accretion is occurring, is somewhat simpler. However, the
larger scale field is distorted close to the star by the stronger field
regions, with the net effect being that the fractional open flux through the
stellar surface is less than would be expected with a dipole magnetic field
model. Finally, we estimate the disc truncation radius, assuming that this
occurs where the magnetic torque from the stellar magnetosphere is comparable
to the viscous torque in the disc.
