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Published online by Cambridge University Press: 07 August 2017
Variability of the emission-line spectra of broad-line AGNs is now a well-established phenomenon (see Peterson 1988 for a review). The rapid variability of the emission lines and the strong correlation between continuum and line fluxes suggests broad-line region (BLR) dimensions too small to be consistent with theoretical estimates based on photoionization equilibrium considerations.