We discuss the problem of the lack of orthogonality between continuum final states obtained by the use of an optical potential and the initial bound states in the description of knockout reactions. Orthogonality is in general lost whenever the description is restricted to a model space with other channels suppressed. The use of suitable effective operators may correct for the orthogonality defect. We obtain an orthogonality condition and evaluate the importance of the orthogonality defect for knockout reactions induced by electromagnetic probes. For (e, e'p) reactions this effect is found irrelevant. For (γ, p) reactions the use of an unfactorized cross section overcomes the difficulty, whereas a large spurious electric dipole effect is found in the usual factorized approximation
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