We investigate the detectability of post-Newtonian higher harmonics from Galactic double white dwarfs in the centihertz band ($\sim 0.01\mathrm{Hz}$). Using a synthetic population, we show that, unlike the quadrupole mode, higher harmonics remain undetectable with the Laser Interferometer Space Antenna (LISA) except for rare nearby systems. In contrast, planned mid-band (decihertz) observatories such as DECIGO and BBO will be able to detect the third harmonic for about 10% of inspiral binaries above $\sim 5\mathrm{mHz}$, enabling statistical constraints on mass ratios. These results highlight the successive roles of LISA and future decihertz missions in establishing a coherent strategy for space-based gravitational wave astronomy.

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• Gravitational waves
• Binary stars