  We present phase-resolved spectroscopy of the bright, eclipsing polar HU Aqr
obtained with high time (~30sec) and spectral (1.6 A) resolution when the
system was in a high accretion state. The trailed spectrograms reveal clearly
the presence of three different line components with different width and radial
velocity variation. By means of Doppler tomography their origin could be
located unequivocally (a) on the secondary star, (b) the ballistic part of the
accretion stream (horizontal stream), and (c) the magnetically funnelled part
of the stream (vertical stream). For the first time we were able to derive a
(near-)complete map of the stream in a polar. We propose to use Doppler
tomography of AM Herculis stars as a new tool for the mass determination of
these binaries. This method, however, still needs to be calibrated by an
independent method. The asymmetric light curve of the narrow emission line
originating on the mass-donating companion star reveals evidence for
significant shielding of 60% of the leading hemisphere by the gas between the
two stars.
