RX J2115.7-5840 - A SHORT-PERIOD, ASYNCHRONOUS POLAR

We report phase-resolved optical polarimetric, photometric and spectro scopic observations of RXJ2115.7-5840 (= EUVE J2115-58.6, Craig 1996) which confirms the system to be a magnetic cataclysmic binary of the p olar (AM Herculis) subclass. The optical light curve is sometimes flat and occasionally displays a pronounced bright phase, reminiscent of t he self-eclipse of a small accretion spot by the revolving white dwarf , as seen in self-eclipsing polars. Our period search reveals ambiguou s results only which can be interpret assuming that the white dwarf is not synchronously rotating with the binary orbit. We find circularly polarized cyclotron radiation with VII ranging from 0% to -15% on one occasion, from -8% to +15% on another occasion. Compared with other po lars, the self-occulted accretion region of RX J21 15.7-5840 had an ex treme red cyclotron spectrum. In addition, the system has an extreme h ard X-ray colour during the ROSAT all-sky survey observation. Both pro perties suggest a low value of the magnetic field strength, and our be st estimate gives B = 11 +/- 2 MG. Due to the absence of significant M -star features in our low-resolution spectra we estimate the minimum d istance to RX J21 15.7-5840 to be d > 250 pc (for an M5(+) secondary s tar).