  Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting
gravitational wave (GW). Orbital motion of some of these binaries are expected
to be observed at low frequency band by space-borne laser interferometric
detectors of GW. The merger remnant may suffer thermonuclear runaway and
explode as type Ia supernova if they are massive enough. As GW sources the
remnants have so far been scarcely studied. Here we propose a new mechanism of
GW emission from DWD merger remnants which may be observed by planned GW
detectors in decihertz frequency band. A remnant is temporarily expected to
have a high degree of differential rotation as a consequence of merger process.
It is then unstable to oscillation modes whose azimuthal pattern speed
coincides with the stellar rotation. We solve eigenvalue problem of
differentially rotating remnants and identify unstable eigenmodes which may be
categorized to inertial modes. The estimate of characteristic strain of GW
shows that they may be detectable within the distance of the Virgo cluster by
planned gravitational wave observatories targeting the decihertz band.
