A NEW ROSAT DISCOVERED POLAR NEAR THE LOWER PERIOD LIMIT RXJ1015.5+0904 IN LEONIS

We have identified the bright, soft, X-ray source RXJ1015.6+0903 detec ted during the ROSAT All-Sky Survey with a new AM Herculis binary (pol ar). Its soft X-ray light curve is typical for a system with a one-pol e accretion geometry and shows a bright phase lasting about 0.75 of th e binary orbit with ROSAT PSPC countrates of up to 10 cts/s. The brigh t phase is interrupted by a 3 min long dip with zero countrate when th e X-ray emission from the accretion region must be absorbed by the acc retion stream crossing our line-of-sight. From the X-ray dip times we derive an orbital period P-orb = 79.87946(7) min making this system an other member of the new group of polars discovered near the theoretica l lower period limit for magnetic CVs. In the optical, the light curve is double-humped and the system varies between V = 16-17 mag in the h igh state. The optical light is highly polarized with the degree of ci rcular polarization varying between -5% and +30% in R and -5% and 22% in I which is a strong indication for the magnetic nature of the syste m. The shape of the optical light curve and the broad optical continuu m component seen as additional light during the bright-phase can be ex plained by cyclotron emission in a magnetic field of similar to 23 MG in the accretion region. At this field strength only the higher cyclot ron harmonics are present in the optical and are smeared to an almost featureless continuum at a viewing angle theta similar to 55 degrees. The orbital inclination of the new ARI Her binary can be constrained t o 50 degrees less than or similar to i less than or similar to 75 degr ees and the mass ratio must be 6.3 < Q = M-1/M-2 < 12.5 where the mass of the secondary star is estimated to M-2 = 0.09 M.. No features of t he late hi-type secondary are present in our low-state optical spectra , providing a lower limit for the distance to RXJ1015.6+0904 of d > 10 0 pc.