  $R$-process enhanced stars with [Eu/Fe]$\geq+0.7$ (so-called $r$-II stars)
are believed to have formed in an extremely neutron-rich environment in which a
rare astrophysical event (e.g., a neutron star merger) occurred. This scenario
is supported by the existence of an ultra-faint dwarf galaxy, Reticulum~II,
where most of the stars are highly enhanced in $r$-process elements. In this
scenario, some small fraction of dwarf galaxies around the Milky Way were $r$
enhanced. When each $r$-enhanced dwarf galaxy accreted to the Milky Way, it
deposited many $r$-II stars in the Galactic halo with similar orbital actions.
To search for the remnants of the $r$-enhanced systems, we analyzed the
distribution of the orbital actions of $N=161$ $r$-II stars in the Solar
neighborhood by using the Gaia EDR3 data. Since the observational uncertainty
is not negligible, we applied a newly-developed {\it greedy optimistic
clustering method} to the orbital actions of our sample stars. We found six
clusters of $r$-II stars that have similar orbits and chemistry, one of which
is a new discovery. Given the apparent phase-mixed orbits of the member stars,
we interpret that these clusters are good candidates for remnants of completely
disrupted $r$-enhanced dwarf galaxies that merged with the ancient Milky Way.
