2-POLE ACCRETION IN THE HIGH-FIELD POLAR RXJ-1938.6-4612

We report the discovery of two systems of cyclotron emisson lines in p hase-resolved low-resolution spectra of the recently discovered AM Her culis binary RXJ 1938.6-4612. The fieldstrengths inferred are B-1 simi lar or equal to 47 MG and B-2 similar or equal to 70-80 MG, which migh t be indicative of a non-dipolar field structure. The latter value is the highest found so far for white dwarfs in cataclysmic variables. We present a spectroscopic ephemeris of sufficient accuracy to connect o ur optical and ROSAT All Sky Survey X-ray observations separated by a few years. Using this, the X-ray bright accretion spot which is visibl e for less than 40% of the binary cycle may be identified with the low -field accretion region. The high-field spot dominates the optical lig htcurve. We find slight evidence for a quadratic term in the ephemeris , P = (-3.4+/-2.1) x 10(-9). Low-state spectra reveal M-star features, Zeeman absorption troughs from the white dwarf and cyclotron emission lines originating in the high-field region. Photospheric Zeeman model s using a slightly decentered dipole along the axis, z(off) similar or equal to 0.1 R(wd), B-pole similar or equal to 60 MG, fit some absorp tion features in the blue spectral range and confirm the results of hi gh-state cyclotron spectroscopy. The implied distance to RXJ 1938.6-46 12 is similar to 150-190 pc.