  We have investigated magnetoelastic effects in multiferroic HoMnO$_3$ below
the antiferromagnetic phase transition $T_N \approx 75$ K by neutron powder
diffraction. The lattice parameter $a$ of the hexagonal unit cell of HoMnO$_3$
decrease in the usual way at lower temperatures and then shows abrupt
contraction below $T_N$ whereas the lattice parameter $c$ increases
continuously with decreasing temperature and shows an anomalous increase below
$T_N$. The unit cell volume decreases continuously with decreasing temperature
and undergoes abrupt contraction below $T_N$. By fitting the background thermal
expansion for a nonmagnetic lattice with the Einstein-Gr\"uneisen equation we
determined the lattice strains $\Delta a$, $\Delta c$ and $\Delta V$ due to the
magnetoelastic effects as a function of temperature. We have also determined
the temperature variation of the ordered magnetic moment of Mn ion by fitting
the measured Bragg intensities of the nuclear and magnetic reflections with the
known crystal and magnetic structure models.
