Variability and Transits in Kepler White Dwarfs

This Thesis undertakes a photometric survey of White Dwarfs observed by the Kepler and Kepler 2 space telescope mission. I determine that between 20-50% of White Dwarfs are variable. The amplitude of such variability varies from between tens of parts per million to those with amplitudes of several percent with periods ranging from hours to several days. No correlation is found between mass and temperature, magnetic fields, atmosphere metal contamination or infra-red excess and variability. The variability is hence likely to be due to a combination of factors including both due to companions as a result of reflection, heating and relativistic beaming and rotational variability such as from spots, magnetic variability and possibly metal contamination. To determine the nature of such variability further spectroscopic observations were undertaken of the photometrically selected White Dwarfs which identified that ~20% were contaminating objects such as sub-dwarfs. Thirty five new White Dwarfs were identified including one new magnetic and two metal contaminated White Dwarfs.The photometric variability survey identified three binary systems. The first is confirmation of an eclipsing double White Dwarf system with a period of 2.3967 hours. Secondly a new White Dwarf – Brown Dwarf system is identified from periodic variability and analysis of the spectroscopic follow up. A period of 68.2 minutes is confirmed making this the shortest known (at the time of writing) non-interacting such system. Finally I report on the discovery of the first known eclipsing sdO-sdO eclipsing binary system with a period of 2.5 hours.