  The nucleus is a correlated open quantum many-body system. The presence of
states that are unbound to particle emission may have significant impact on
spectroscopic properties of nuclei, especially those close to the particle drip
lines. In the framework of the continuum shell model in the complex
momentum-plane, we discuss salient effects of the continuum coupling on the
spectroscopic factors, one-neutron overlap integrals, and energies of excited
states in $^{6}$He and $^{18}$O. We demonstrate the presence of
non-perturbative threshold effects in many-body wave functions of weakly-bound
and unbound states.
