  We investigate the magnetic geometry of the active G8 dwarf xi Bootis A, from
spectropolarimetric observations obtained in 2003 with the MuSiCoS echelle
spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du Midi,
France). We repeatedly detect a photospheric magnetic field, with periodic
variations consistent with rotational modulation. Circularly polarized (Stokes
V) line profiles present a systematic asymmetry, showing up as an excess in
amplitude and area of the blue lobe of the profiles. A direct modeling of
Stokes V profiles suggests that the global magnetic field is composed of two
main components, with an inclined dipole and a large-scale toroidal field. We
derive a dipole intensity of about 40 G, with an inclination of 35 degrees of
the dipole with respect to the rotation axis. The toroidal field strength is of
order of 120 G. A noticeable evolution of the field geometry is observed over
the 40 nights of our observing window and results in an increase of the field
strength and of the dipole inclination.
  This study is the first step of a long-term monitoring of xi Bootis A and
other active solar-type stars, with the aim to investigate secular fluctuations
of stellar magnetic geometries induced by activity cycles.
