The Evolution of Magnetic CVs

About one-fourth of all cataclysmic variables (CVs) contain white dwarfs with magnetic fields that are sufficiently strong (B≳5 × 104 G) to disrupt the accretion disk, either partially or completely, and are known as magnetic CVs (Chanmugam 1992, Ritter & Kolb 1995). Two classes of magnetic CVs have been distinguished: the synchronous systems in which the spin period of the white dwarf P spin and the orbital period P orb are equal to within about 2 %, and the asynchronous systems where the spin and orbital periods are more widely different (Sparks et al 1995). The former are also known as AM Herculis binaries or polars (Cropper 1990). Their magnetic field strengths B have been determined from Zeeman and cyclotron spectroscopy, and from cyclotron emission models for the optical/IR polarization they emit (Chanmugam and Dulk 1981; Meggitt and Wickramasinghe 1982). With the recent discovery of the probably synchronized system 1H1752+08 with B ≈ 7 MG (Ferrario et al. 1994), the range of their magnetic fields is 7 MG – 70 MG (Chanmugam 1992). In addition, most of these binaries have P orb ∓ 4 hours. The resulting short orbital separation and strong magnetic field, enforces the synchronous rotation of the white dwarf (e.g., Joss, Katz and Rappaport 1979; Campbell 1990; King et al. 1990; Wu & Wickramasinghe 1993). The number of AM Her binaries has more than tripled in the last few years to over 50 (see e.g., Beuermann 1995) as result of the large number of them discovered by ROSAT from their softer X-ray spectra.