White dwarf-red dwarf binaries in the Galaxy

This PhD thesis shows several studies on white dwarf - red dwarf binaries. White dwarfs are the end products of most stars and red dwarfs are normal hydrogen burning low-mass stars. White dwarf - red dwarf binaries are both blue (white dwarf) and red (red dwarf). Together with the fact that they are intrinsically equally bright, these binaries stand out in any colour-colour diagram. These studies have mainly used the largest astronomical database available at present, the Sloan Digital Sky Survey. This database contains 287 million objects in 5 optical filters and spectroscopy for about 1% as well. The first study in this thesis describes the discovery of 15 rare white dwarf - red dwarf binaries. The second study involves the bright, eclipsing binary DE Canum Venaticorum for which a large number of photometric and spectroscopic observations are analysed. The last part of this thesis describes the selection and analyzation of a large sample of white dwarf - red dwarf binaries in the Sloan Digital Sky Survey, which are selected on the basis of their colours and apparent motion in the sky. With the use of spectral models the available spectra for these objects are fitted to determine the white dwarf temperature and secondary spectral type. After that, the whole photometric sample of binaries was taken and classified on the basis of their colours alone. With the use of a simulated galactic model the space density for white dwarf - red dwarf binaries is determined. This is the first time that this kind of study has been done