Short-period Galactic double white dwarf (DWD) systems will be observable both in visible light through photometric monitoring and in mHz-range gravitational waves (GWs) with forthcoming space-based laser interferometry such as the Laser Interferometer Space Antenna. When only photometric variability is used to measure DWD intrinsic properties, there is a degeneracy between the chirp mass and binary tidal interaction, as the orbital frequency time derivative is set by both GW radiation and tides. Without expensive radial velocity data from spectroscopic monitoring, this degeneracy may be lifted in principle by directly measuring the second time derivative of the orbital frequency through photometric monitoring over an ultralong time baseline. Alternatively, the degeneracy can be removed by exploiting information in both photometric variability and the coherent GW waveform. Investigating both approaches, we find that direct measurement of the second time derivative is likely infeasible for most DWDs, while the multimessenger method will disentangle measurements of the chirp mass and the binary moments of inertia for a large sample of tidally locked systems. The latter information will enable empirical tests of white dwarf structure models with finite temperature effects.

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• Binary stars
• Electromagnetic radiation astronomy
• Gravitational wave detection
• Gravitational wave detectors