Double White Dwarf Binaries in SDSS-V DR19: The Discovery of a Rare DA+DQ White Dwarf Binary with a 31 hr Orbital Period

Binaries of two white dwarfs (WDs) are an important class of astrophysical objects that help us reconstruct the complex processes involved in stellar evolution and are theorized to lead to Type Ia supernovae. We report the discovery of SDSS J090618.44+022311.6, a rare post-common-envelope binary of a hydrogen atmospheric DA WD and a DQ WD that shows carbon absorption features and is only the fourth such binary known. We combine the available spectroscopic, photometric, and radial velocity data to provide a self-consistent model for the binary and discuss its history as a binary DA+DQ. The system has a period of 31.17 hr with masses of 0.39 M_⊙ for the DA WD and 0.49 M_⊙ for the DQ WD. The corresponding cooling ages point to a scenario where the lower-mass DA WD forms first when the more massive of the two progenitor stars transfers mass on to the companion after the main sequence. The companion later evolves to form the more massive DQ WD. The system has a merger timescale of 450 Gyr and will lead to the formation of a massive WD. We find that the stellar properties of all four known post-common-envelope DA+DQ systems are in a narrow range. The dynamical mass measurement of the DQ WD in this work supports the existing hypothesis that DQ WDs preferentially evolve from low-mass DB WDs.