  We propose that the NMR line shape on the chain Cu in the stoichiometric
high-$T_c$ superconductor Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ is determined by the
magnetization induced on Cu near O vacancies, due to strong magnetic
correlations in the chains. An unrestricted Hartree-Fock calculation of a
coupled chain-plane Hubbard model with nearest-neighbor d-wave pairing
interaction shows that the broadening of NMR lines is consistent with
disorder-induced magnetization at low temperatures. In addition, we give a
possible explanation of the anomalous bimodal line shape observed at high
temperatures in terms of nonuniform Cu valence in the chains. The proximity
between chains and CuO plane induces anisotropic magnetization on the planar
Cu, and broadens the plane NMR lines in accordance with that of the chain
lines, in agreement with experiment. We discuss implications of the model for
other experiments on underdoped YBCO.
