  Oxygen NMR is used to probe the local influence of nonmagnetic Zn and
magnetic Ni impurities in the superconducting state of optimally doped high Tc
YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to
that evidenced above Tc, with a typical extension xi=3 cell units for Zn and
xi>=3 for Ni. In addition, Zn is observed to induce a local density of states
near the Fermi Energy in its neighbourhood, which also decays over about 3 cell
units. Its magnitude decreases sharply with increasing temperature. This allows
direct comparison with the STM observations done in BiSCO.
