Upper panel: number fractions of electrons Y_e (solid lines) and muons Y_μ (dashed lines) in the core of a neutron star, relative to the number of nucleons, as functions of nucleon number density n_b for the three EoSs indicated near the curves. Lower panel: differences between the fit (9) and numerically computed tables for Y_e and Y_μ, plotted against n_b. The vertical dotted lines show the position of the boundary between the crust and the core (n_cc in Table 7).