X-ray observations of cataclysmic variable stars with the XMM-Newton space observatory

Cataclysmic variables are compact binary star systems in which matter is transferred from a Roche lobe filling main-sequence star onto a white dwarf. I present new findings obtained from X-ray observations of fifteen cataclysmic variables with the XMM-Newton space observatory.;For the AM Her type binaries DP Leo and WW Hor, I study orbital variations of the X-ray emission to derive the structure and geometry of the accretion region on the white dwarf. I find that the orbital period of DP Leo is decreasing significantly faster than expected for angular momentum loss via gravitational radiation. The AM Her type binaries UZ For, VV Pup, and V393 Pav were observed during unusual states of very low brightness and strong X-ray and ultraviolet flaring. The flaring was caused by irregular mass transfer due to solar flares or coronal mass ejections on the companion stars and provides evidence for highly active stellar atmospheres.;For a sample of nine dwarf novae, I investigate the structure of the boundary layer between accretion disk and white dwarf and study some general properties of dwarf novae at low accretion rates. I find that the X-ray emitting plasma in the boundary layer is optically thin and closely resembles an isobaric cooling flow. I find no evidence of underluminous boundary layers in the quiescent dwarf novae. The boundary layers in all nine dwarf novae rotate significantly slower than the inner accretion disks. These findings have important implications for models of accretion in quiescent dwarf novae.;I estimate elemental abundances on the companion stars and demonstrate that an X-ray spectral analysis can be used to determine whether unusual white dwarf abundances are the result of a thermonuclear runaway or accretion from the companion star. In the quiescent dwarf nova VW Hyi. I discovered a 60 second quasi-periodic X-ray oscillation and a correlation between the X-ray and ultraviolet variability observed on a 1500 second time scale. Orbital X-ray variations in VW Hyi indicate warping of the inner disk. Based on the X-ray spectra, I reclassify EI UMa, previously thought to be a dwarf nova, as an intermediate polar.