  The recent neutron-scattering data for spin-wave dispersion in $\rm Ho Mn
O_3$ are well described by an anisotropic Hubbard model on a triangular lattice
with a planar (XY) spin anisotropy. Best fit indicates that magnetic
excitations in $\rm Ho Mn O_3$ correspond to the strong-coupling limit $U/t >
\sim 15$, with planar exchange energy $J=4t^2/U \simeq 2.5$meV and planar
anisotropy $\Delta U \simeq 0.35$meV.
