  Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05Msun are
basically believed to harbour oxygen-neon (ONe) cores. Recently, Gaia data
reveals an enhancement of UMWDs on Hertzsprung-Russell diagram (HRD), which
indicates that extra cooling delay mechanism such as crystallization and
elemental sedimentation may exist in the UMWDs. Further studies suggested that
a portion of UMWDs should have experienced pretty long cooling delays, implying
that they are carbon-oxygen (CO) WDs. However, the formation mechanism of these
UMCOWDs is still under debate. In this work, we investigated whether the merges
of massive CO WDs with helium WDs (He WDs) can evolve to UMCOWDs. By employing
stellar evolution code MESA, we construct double WD merger remnants to
investigate their final fates. We found that the post-merger evolution of the
remnants are similar to R CrB stars. The helium burning of the He shell leads
to the mass growing of the CO core at a rate from 2.0*10^-6 to 5.0*10^-6
Msun/yr . The final CO WD mass is influenced by the wind-mass-loss rate during
the post-merger evolution, and cannot exceed about 1.2Msun. The remnants with
core mass larger than 1.2Msun will experience surface carbon ignition, which
may finally end their lives as ONe WDs. Current results implies that at least a
portion of UMWDs which experience extra long cooling delay may stem from
merging of CO WDs and He WDs.
