  We report phase-resolved optical polarimetric, photometric and spectroscopic
observations of RX J2115.7-5840 (= EUVE J2115-58.6, Craig 1996) which confirms
the system to be a magnetic cataclysmic binary of the polar (AM Herculis)
subclass. The optical light curve is sometimes flat and occasionally displays a
pronounced bright phase, reminiscent of the self-eclipse of a small accretion
spot by the revolving white dwarf, as seen in self-eclipsing polars. Our period
search reveals ambiguous 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 V/I ranging from 0% to -15% on
one occasion, from -8% to +15% on another occasion. Compared with other polars,
the self-occulted accretion region of RXJ2115 had an extreme red cyclotron
spectrum. In addition, the system has an extreme hard X-ray colour during the
ROSAT all-sky survey observation. Both properties suggest a low value of the
magnetic field strength, and our best estimate gives B = 11+-2 MG. Due to the
absence of significant M-star features in our low-resolution spectra we
estimate the minimum distance to RXJ2115 to be d > 250pc (for an M5+ secondary
star).
