1998A&A...331..262B

We have identified the bright, soft, X-ray source RX J1015.6+0904 detected during the ROSAT All-Sky Survey with a new AM Herculis binary (polar). Its soft X-ray light curve is typical for a system with a one-pole accretion geometry and shows a bright phase lasting about 0.75 of the binary orbit with ROSAT PSPC countrates of up to 10cts/s. The bright phase is interrupted by a 3min long dip with zero countrate where the X-ray emission from the accretion region must be absorbed by the accretion 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 another member of the new group of polars discovered near the theoretical lower period limit for magnetic CVs. In the optical, the light curve is double-humped and the system varies between V=16-17mag in the high state. The optical light is highly polarized with the degree of circular 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 system. The shape of the optical light curve and the broad optical continuum component seen as additional light during the bright-phase can be explained by cyclotron emission in a magnetic field of ∼23MG in the accretion region. At this field strength only the higher cyclotron harmonics are present in the optical and are smeared to an almost featureless continuum at a viewing angle θ∼55°. The orbital inclination of the new AM Her binary can be constrained to 50°≲i≲75° and the mass ratio must be 6.3<Q=M₁/M₂<12.5 where the mass of the secondary star is estimated to M₂=0.09M_☉ . No features of the late M-type secondary are present in our low-state optical spectra, providing a lower limit for the distance to RX J1015.6+0904 of d>100pc