The DBL Survey II Towards a mass-period distribution of double white dwarf binaries

Double white dwarf (DWD) binaries are an important remnant of binary evolution as they are possible Type Ia supernova progenitors and strong sources of gravitational waves in the low-frequency regime. The double-lined double white dwarf (DBL) survey searches for compact DWDs where both stars can be spectrally disentangled. Candidates are identified by being overluminous compared to the cooling sequence of a typical-mass, single white dwarf. In this second instalment of the DBL survey, we present full orbital solutions of 15 DWD binaries from our ongoing campaign to accurately measure a magnitude-limited mass–period distribution. For the first time, 12 of these systems are fully solved. A long-standing bias in the full population has been evident, favouring systems with orbital periods up to a few hours, with little exploration of the majority of the compact DWD population, whose orbital period distribution centres at approximately 20 h. The 15 systems in this study span the orbital period range 5 < P_orb < 75 h, significantly augmenting the number of well-characterized systems over these periods, and in general have two similar mass stars combining to ≈1.0 M_☉⁠. We witness that the orbitally derived mass ratios generally show excellent agreement with those deduced from atmospheric fits to double-lined spectra in previous work, emphasizing the power of wide-scale spectroscopic surveys to efficiently locate the highest-mass, double-lined DWDs in the local Galaxy.