During the low optical brightness states of AM Her systems (polars) when accretion has declined to a very low value, the underlying magnetic white dwarf photosphere can be modelled without the complication of thermal bremstrahlung and cyclotron emission from the luminous accretion column. The far ultraviolet spectra can be modelled with high gravity Solar composition photospheres. In this way, I present new temperatures and the first chemical abundance estimates for the white dwarfs in three selected polars from the IUE NEWSIPS archive. For the white dwarf in V834 Cen with Teff = 16,000K, Si/H = 0.1 Solar, C/H = 0.5 Solar, for BY Cam, Teff = 17,000K, Si/H = 0.1, C/H = 5 Solar and for RX J1313-32, Teff = 22,000K, Si/H = 0.1 Solar, C/H = 0.1 Solar. The temperature distribution of 24 white dwarfs in polars with known temperatures above and below the period gap is compared with the distribution of the white dwarf temperatures in dwarf novae during quiescence. In both cases, the magnetic white dwarfs in polars are significantly cooler than the non-magnetics. For all CV white dwarfs, magnetic and non-magnetic with Teff < 12,500K, 91% of the objects (10 out of 11) are magnetics in polars. This suggests that long term accretion heating and cooling of white dwarfs in polars differs from the effects of long term accretion in non-magnetic disk accretors.

We investigate the spin evolution of the unusual magnetic CV AE Aqr. As a prototype for a potentially large population of CVs subject to a thermally unstable phase of mass transfer, understanding its future is crucial. We present a new definition of the magnetospheric radius in terms of the white dwarf’s spin period, and use this along with numerical simulations to follow the spin evolution of AE Aqr. We also present preliminary SPH results suggesting the existence of a stable propeller state. These results highlight the complexity of MCVs and may provide an improved understanding of the evolution of all types of CVs.

We have observed several magnetic cataclysmic variables located in the range between 2 and 3 hours, known as the period gap. This work was prompted by the recent discovery of RXJ1554.2+2721. It has 2.54 hours orbital period and shows almost pure cyclotron continuum in a low luminosity state, similar to HS1023+3900, HS0922+1333 and RBS206. These are low accretion rate polars (LARPs) known to have mass transfer rates of order of a few 10-13M⊙/year. The aim of the study was to find out, if magnetic systems filling the period gap are in any way different from their counterparts outside that range of periods. The only significant difference we encounter is a much higher number of asynchronous magnetic systems towards longer periods than below the gap.

Recent quasar surveys have identified several new magnetic cataclysmic variables accreting at remarkably low rates, ~10−13M⊙ yr−1. The new discoveries raise questions regarding selection effects that may influence the current sample and the traditional evolutionary conclusions that have been drawn. This paper reviews the techniques that have been used to identify polars, including a summary of the optical polarization as a function of accretion rate and magnetic field strength. The new, low-ṁ systems accrete without a shock and cooling is dominated by optical cyclotron emission in very well-defined harmonics. These binaries, which have been found to accrete at far below the rate corresponding to angular momentum loss by gravitational radiation, also appear to contain unusually cool white dwarfs, suggesting that they are either at an advanced age or in unusual evolutionary states.

The first two years of the Sloan Digital Sky Survey have discovered over 50 new cataclysmic variables, of which four are confirmed Polars, two others are likely candidates and several others are good candidates for Intermediate Polars or SW Sex stars. Several systems have unusual characteristics, including large amplitude, narrow cyclotron humps indicating a very low temperature shock on a cool white dwarf, or very deep eclipses. Although SDSS is finding a variety of systems, it is ideal for picking up the low mass transfer rate magnetic systems missed in X-ray surveys and in past optical surveys with brighter limits. Thus, it is providing us with a clearer picture of the range of conditions existing in magnetics CVs.

An overview is given of all the magnetic CVs found in the UCT CCD CV Survey (Woudt & Warner 2001, 2002, 2003a). We have identified eight new candidate Intermediate Polars (IP), of which six are classical novae (RR Cha, DD Cir, AP Cru, V697 Sco, V373 Sct, and RXJ1039.7-0507). The two other candidate IPs are Aqr1 (2236+0052) and RX J0944.5+0357. In addition, there are two probable Polars, namely V351 Pup (= Nova Puppis 1991) and FIRST J102347.6+003841.

We show that VY Scl stars must be magnetized in order to account for the absence of outbursts during their low and intermediate states. Absence of outbursts during low states requires only rather low magnetic moments but in systems in which the drops and rises of luminosity are slower than it takes for the accretion disc to adjust viscously to the variation in mass-transfer rate preventing outbursts require magnetic moments of Intermediate Polars. We discuss some evolutionary aspects of this conclusion.

We present fast spectroscopy of RX J1548-45 and BT Mon acquired with EFOSC-ESO 3.6 m in March & April 2002, with the aim to detect for the first time spectral variability at the white dwarf rotation.

A short serendipitous XMM–Newton observation of BL Hyi is presented.

I discuss the effect of accretion on the magnetic field of an accreting white dwarf. Whereas the magnetic fields of isolated white dwarfs are not expected to change significantly with time, I show that if an accreting white dwarf increases in mass at a rate > 1–5 × 10−10MΘ yr–1, accretion overcomes ohmic diffusion and has a significant effect on the field structure. I discuss the implications of this result for observed systems. In particular, accretion induced field evolution may provide the missing evolutionary link between the strong field, slowly accreting AM Her systems and the weak field, more rapidly accreting intermediate polars.

We report first results of our systematic investigation of the magnetic field structure of rotating single magnetic white dwarfs and of white dwarfs in magnetic cataclysmic variables. The global magnetic field distributions on the isolated white dwarf HE 1045-0908 and the accreting white dwarfs in EF Eri and CP Tuc have been derived from phase-resolved flux and polarization spectra obtained with FORS1 at the ESO VLT using the systematic method of Zeeman tomography.

Stokes imaging uses polarimetric observations in order to image indirectly the accretion region on the surface of the white dwarf in magnetic cataclysmic variables (MCVs). Doppler tomography uses spectroscopic observations in order to gain insights into the velocity dynamics of the secondary star, and the ballistic and magnetically confined parts of the accretion stream. Until now, both of these techniques have been applied separately. We present and compare the results of applying both techniques to simultaneous spectroscopic and polarimetric observations of MCVs.

Seventeen examples are given of Cataclysmic Variable (CV) stars possessing both Dwarf Nova Oscillations (DNOs) and Quasi-Periodic Oscillations (QPOs). These form an extension of the X-Ray Two-QPO correlation to frequencies three orders of magnitude lower. We draw attention to the existence of a second type of DNO in CVs, which is probably caused by magnetically channelled accretion onto the white dwarf.

Multiwavelength observations of polars are essential for developing the big picture of these systems, particularly to gain understanding of the relevant accretion-induced heating and cooling processes. Eclipsing polars are prime targets for such studies since different radiation processes can be disentangled by observations with high-time resolution. We present a preliminary combined analysis of space-based observations (XMM-Newton, ROSAT, HST) with ground-based high-speed photometry (MCCP, OPTIMA, ULTRACAM) of DP Leo, HU Aqr and UZ For. We determine the location and extent of different emission components and find secular and short-term changes in the accretion geometries. We find displaced optical and X-ray emission regions in DP Leo and HU Aqr as well as mini-bursts and accretion arcs of variable size in HU Aqr. We report marked changes in the X-ray eclipse length of UZ For between high and low states.