  We have started a systematic study of the field topologies of magnetic single
and accreting white dwarfs using Zeeman tomography. Here we report on our
analysis of phase-resolved flux and circular polarization spectra of the
magnetic cataclysmic variables BL Hyi and MR Ser obtained with FORS1 at the ESO
VLT. For both systems we find that the field topologies are more complex than a
dipole or an offset dipole and require at least multipole expansions up to
order l = 3 to adequately describe the observed Zeeman features and their
variations with rotational phase. Overall our model fits are in excellent
agreement with observations. Remaining residuals indicate that the field
topologies might even be more complex. It is, however, assuring that the global
characteristics of our solutions are consistent with the average effective
field strengths and the halo field strengths derived from intensity spectra in
the past.
