  We investigate the low temperature X-band electron paramagnetic resonance
(EPR) of YBa2Cu3O6 compounds with x = 6.0 doped with Dy3+, Tb3+, and Nd3+. The
EPR spectra of Dy3+ and Tb3+ have been identified. The EPR of Tb3+ is used also
to study the effect of suppression of high Tc superconductivity. The EPR of
Nd3+ is probably masked by the intense resonance of Cu2+. All experimental EPR
results compare well with theoretical estimations.

Superconductivity;Strongly Correlated Electrons