  The bulk viscosity of the neutron star matter due to the direct Urca
processes involving nucleons, electrons and muons is studied taking into
account possible superfluidity of nucleons in the neutron star cores. The cases
of singlet-state pairing or triplet-state pairing (without and with nodes of
the superfluid gap at the Fermi surface) of nucleons are considered. It is
shown that the superfluidity may strongly reduce the bulk viscosity. The
practical expressions for the superfluid reduction factors are obtained. For
illustration, the bulk viscosity is calculated for two models of dense matter
composed of neutrons, protons,electrons and muons. The presence of muons
affects the bulk viscosity due to the direct Urca reactions involving electrons
and produces additional comparable contribution due to the direct Urca
reactions involving muons. The results can be useful for studying damping of
vibrations of neutron stars with superfluid cores.
