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, PSR 1055--52, and RX J1856--3754, in our model) and on the rapid cooling of massive NSs. They affect, however, the moderate 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.

Page created on March 31, 2002.