  Mirror Dark Matter (MDM) has been considered as an elegant framework for a
particle theory of Dark Matter (DM). It is supposed that there exists a dark
sector which is mirror of the ordinary matter. Some MDM models allow particle
interactions mirror and ordinary matter, in addition to the gravitational
interaction. The possibility of neutron to mirror neutron transition has
recently been discussed both from theoretical and experimental perspectives.
This paper is based on a previous work in which we obtained stringent upper
limits on the possibility of converting neutrons to mirror neutrons in the
interiors of neutron stars, by using timing data of binary pulsars. Such a
transition would imply mass loss in neutron stars leading to a significant
change of orbital period of neutron star binary systems. The observational
bounds on the period changes of such binaries, therefore put strong limits on
the above transition rate and hence on the neutron -- mirror-neutron mixing
parameter $\epsilon'$. Our limits are much stronger than the values required to
explain the neutron decay anomaly via $n-n'$ mixing.
