  We extend our modeling of cooling of superfluid neutron stars by including
the production of muons in the core, in addition to neutrons, protons, and
electrons. The results are confronted with observations of middle-aged isolated
NSs. Muons have little effect on the hydrostatic structure of NSs, on the slow
cooling of low-mass NSs (RX J0822--43 and PSR 1055--52 in our model) and on the
rapid cooling of massive NSs. They affect, however, the moderately fast cooling
of medium-mass NSs (1E 1207--52, RX J0002+62, PSR 0656+14, Vela, and Geminga)
and shift appreciably the mass range of these NSs to lower masses, which is
important for correct interpretation of the observations. Moreover, the effects
of muons can accurately be reproduced by a simple renormalization of NS models
with no muons in the NS cores.
