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Abstract:

We carried out medium resolution spectroscopy of close, detached white dwarf - red dwarf binaries without planetary nebulae and cataclysmic variables in the infra-red K-band (2.3 - 2.4 mum). We have detected the (CO)-C-12 bands at 2.3535 mum and 2.3829 mum and, for the first time, the (CO)-C-13 bands at 2.3448 mum and 2.3739 mum from the secondary stars. Comparison with synthetic spectral profiles of these lines indicate that the C-12/C-13 isotopic ratio in these binaries are different from solar. Our preliminary estimates of the isotopic ratios agree well with the theoretical predictions from the common-envelope binary evolution model by Marks et al. (1997). Thus we may have the first direct proof that these objects have indeed undergone common-envelope evolution.

Abstract:

The use of Type Ia Supernovae (SNe Ia) as distance indicators relies on the calibration of their brightness using the observed property that brighter SNe have broader, slower light curves. Among possible causes are different masses of the progenitor white dwarfs or different opacities. We construct Chandrasekhar-mass models which synthesize different amounts of Ni-56, and compute their bolometric light curves and spectra. Since opacity in SNe Ia at early times is due mostly to spectral lines, and Fe-peak ions have many more lines than lighter ions, the opacity depends on the synthesized Ni-56 mass and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions reproduce the relation between brightness and decline rate. The synthetic spectra compare well with spectra of spectroscopically normal SNe Ia which cover a similar Deltam(15)(B) range. The change in colour of the spectra between maximum and two weeks later allows the observed relation between M-B(Max) and Deltam(15)(B) to be reproduced.

Abstract:

I outline a model that gives the mass-loss rate and the magnetic torque on a magnetically active star, as functions of only the mass, luminosity, radius and rotation period. With tidal friction to enforce corotation, this can be applied to a number of types of binary, including contact binaries, RS CVn binaries, Algols, CVs and LMXBs.

Abstract:

We have developed a new method to detect starspots on the surface of the secondary star in cataclysmic variables. We demonstrate that it is a good method, by deducing the host star spectral type, the temperature and percentage coverage of spots, of two U Gem type dwarf novae; SS Cyg and V426 Oph. We support this detection through examining two known spotted stars. This is the first detection of starspots on the cool secondary star in cataclysmic variables and it supports magnetic braking as a method by which such systems evolve.

Abstract:

As presented by Nice et al. (2000), long-term timing of the eclipsing binary PSR B1744-24A shows that the orbital period of this system decreases with a time-scale of only similar to200 Myr. To explain the orbital period decay, which is much faster than that predicted by emission of gravitational waves only (similar to1000 Myr), we propose that the orbital evolution of this system is also driven by magnetic braking. If magnetic braking is to explain the rapid decay of the orbit, then the factor A characterizing the effectiveness of the dynamo action in the stellar convection zone in the magnetic stellar wind formula must be equal to 1.