  Cubic Y$_{2}$Ti$_{2}$O$_{7}$ single crystals doped with Er$^{3+}$ and
Yb$^{3+}$ ions have been studied by the methods of electron spin resonance
(ESR) and selective laser spectroscopy. ESR spectra exhibit signals from
rare-earth ions that substitute for yttrium ions in sites with local trigonal
symmetry. The $g$ tensor components are determined. The results of optical
investigations indicate that impurity centers of several types are formed; the
sublevel energies of the ground and excited multiplets of these centers are
found. Among the great variety of detected optical centers, the centers that
dominate in the formation of ESR spectra are discriminated. An analysis of the
experimental data using the exchange-charge model have made it possible to
determine the sets of parameters of the crystal field for Er$^{3+}$ and
Yb$^{3+}$ ions substituting Y$^{3+}$ ions in regular crystallographic sites in
pyrochlore Y$_{2}$Ti$_{2}$O$_{7}$.
