Program: GN-2015B-Q-67

Type Ia supernovae are cosmic beacons that were crucial for the identification of dark energy, propelling cosmology into a new era. Yet, we are still unable to explain the precise origin of type Ia supernovae, the very tool that led to that discovery. In a recent study, we have shown it to be very likely that the accreting white dwarfs in cataclysmic variables grow in mass - in contradiction to widely accepted theoretical models predicting mass erosion as a consequence of classical nova eruptions. To corroborate our results, we are now in the process of doubling the number of accurate CV white dwarf masses, obtaining Ly-alpha spectroscopy of 33 CVs as part of a 122-orbit HST/COS program. Determining the white dwarf masses to a few per cent requires additional, dynamical constraints, and we apply here for phase-resolved GMOS observations to measure the radial velocities and the rotational broadening of the low-mass companion stars in two of our HST targets.