  We describe the design of a gravitational wave timing array, a novel scheme
that can be used to search for low-frequency gravitational waves by monitoring
continuous gravitational waves at higher frequencies. We show that observations
of gravitational waves produced by Galactic binaries using a space-based
detector like LISA provide sensitivity in the nanohertz to microhertz band.
While the expected sensitivity of this proposal is not competitive with other
methods, it fills a gap in frequency space around the microhertz regime, which
is above the range probed by current pulsar timing arrays and below the
expected direct frequency coverage of LISA. The low-frequency extension of
sensitivity does not require any experimental design change to space-based
gravitational wave detectors, and can be achieved with the data products that
would already be collected by them.
