POLARIMETRY OF THE EXCEPTIONALLY LONG-PERIOD ECLIPSING POLAR RX J0515.6+0105

Photopolarimetric observations of RX J0515.6 + 0105 confirm that this system is a polar (i.e. an AM Herculis system). Circular polarization, varying from -2 to 2 per cent, was measured over an interval of seven nights. Furthermore, a periodogram analysis of the data reveals a 7.9 9-h period, consistent with the photometrically determined orbital per iod. This makes RX J0515.6 + 0105 the longest period polar, exceeding the erstwhile longest period system by similar to 3.3 h. Although the degree of linear polarization is low, there is some evidence for a lin ear polarization 'spike' near orbital phase 0.5, corresponding to when the circular polarization changes sign. Also, the maximum and minimum values of the circular polarization are seen to coincide with the int ensity maxima. These characteristics are evidence for two regions of c yclotron emission of differing polarity, and argue in favour of cyclot ron beaming as the likely explanation for the prominent humps in the l ight curves. The near-zero circular polarization at eclipse implies th at the cyclotron emitting region is offset from the line of centres of the stars by similar to 90 degrees. The presence of likely cyclotron spectral features implies a magnetic field strength of less than or si milar to 61 MG, which is well below the value inferred from Patterson' s synchronism criterion. This is taken as evidence for a much lower ac cretion rate than the empirical M(P) relation would imply.