New low accretion rate magnetic binary systems and their significance for the evolution of cataclysmic variables

Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey ( SDSS) brings to six the total number of X-ray-faint, magnetic accretion binaries that accrete at rates. M <= 10(-13) M-circle dot yr(-1), or < 1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post common-envelope systems whose orbits have not yet decayed to the point of Roche lobe contact. They are premagnetic cataclysmic variables, or pre-Polars. The systems exhibit spin-orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a "magnetic siphon." Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods > 3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors ( in addition to the asynchronous intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.