Mass transfer and hydrogen burning in white dwarf binaries

The thesis is devoted to a study of mass transfer, non-explosive hydrogen burning and the effects of the magnetic field in cataclysmic variables (CVs) in the context of their evolution and possible paths to Supernovae type Ia. I focused on the observational properties of CVs hosting massive white dwarfs (WDs) and/or accreting at high rate. My aims were to detect signatures of quiescent hydrogen burning, follow novae as they settle into quiescence and to assess whether the WD magnetic field alters secular evolution and the outcome of the nova explosions. The thesis consists of three chapters. The first one focuses on observations of VY Scl-type nova-like systems, or "anti-dwarf novae'', exploring the idea that they can burn hydrogen during their optical low states. I analysed all the available archival X-ray and UV observations, in both, their high and their low states, and found that the proposed hydrogen burning at high atmospheric temperatures is ruled out. VY Scl-type stars cannot be Supernova type Ia progenitors since they either burn hydrogen but have too low-mass WDs or undergo rare nova explosions, expelling more material than was accreted. In the second chapter I investigate two post-novae with massive WDs, confirming the magnetic nature of one of them, and revealing that the second one is an intermediate polar candidate. I also show that a nova explosion in a magnetic system can leave an imprint on the surface of a WD, detectable in soft X-rays for several years after the explosion. It may be due to residual localized hydrogen burning, but a more likely explanation is a temperature gradient in the WD atmosphere. The last chapter represents monitoring of the old magnetic nova GK Per performed with Swift, Chandra and NuSTAR telescopes during its recent dwarf nova outburst. The main goal was to study the effects of increased mass transfer through the disk in a magnetic system. I measured the WD spin-up rate, localized the emission sites of different spectral components, revealed shrinking of the inner radius of accretion disk and redistribution of the accretion energy as the mass transfer grows