RXJ2115-5840: confirmation of a new near-synchronous Polar

Following the suggestion of Schwope et al. that the magnetic cataclysmic variable RX J2115--5840 may be a near-synchronous polar, we obtained optical polarimetry of this system over a 2-week period. From a power spectrum of the circular polarimetry data we determine that the spin period of the white dwarf and the binary orbital period differ by 1.2 per cent. RX J2115--5840 is thus the fourth near-synchronous polar and has the shortest spin--orbit beat period: 6.3 d. By folding the data on spin, beat and orbital periods we find evidence that the accretion stream is directed towards opposite magnetic poles as the stream precesses around the white dwarf on the spin--orbit beat period. The phasing requires that the accretion flow must be directed on to the same magnetic field line at all spin--orbit beat phases, implying that at some phases the flow must follow a path around the white dwarf before accreting. This is difficult to reconcile with simple views of how the accretion stream attaches on to the magnetic field of the white dwarf.