Gravitational Wave Radiation of Double Degenerates with Extremely
  low-mass WD companions
  Double Degenerate systems (DDs) are supposed to be significant gravitational
wave (GW) sources for future space-based gravitational-wave detectors, e.g.,
Laser Interferometer Space Antenna (LISA). Recently, one type of DDs with
Extremely low-mass WD (ELM WD; $\leq 0.30\; M_\odot$) companions has been
largely found in the ELM Survey. They have very short orbital periods and are
therefore important sources for LISA detection. Besides, due to the thick
envelope of ELM WDs compared with massive WDs (e.g. CO WDs), they are much
easier to be found by the combination of electromagnetic (EM) and GW
observations. In this paper, we first obtain the population of ELM WDs in DDs
with considering the detailed evolutionary tracks of ELM WDs, and then analyse
the GW radiation of these systems. We found that about $6\times10^3$ sources
could be solely detected by LISA, including $\sim2\times10^3$ chirping sources,
and $\sim13$ ($\sim107$) more sources are expected to be detected by both LISA
and ELM Survey (Gaia).
