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We have monitored spectrophotometrically a subsample (28) of the Palomar-Green Bright Quasar Sample for two years, in order to test for correlations between continuum and emission-line variations and to determine the timescales relevant to mapping the broad-line regions of these high-luminosity AGNs. Half of the quasars monitored underwent continuum ($\sim 5000$ \AA) variations with amplitudes in the range 20-60\%, and the rest varied less. The rise and fall time for the continuum variations is typically 0.5-2 years. In most of these objects we detect correlated variations in the broad $H\alpha$ and $H\beta$ emission lines. The amplitude of the line variations is usually 2-4 times smaller than the optical continuum fluctuations. We present light curves and analyze spectra for six of the variable quasars. The lines respond to the continuum variations with a lag that is smaller than or comparable to our typical sampling interval (a few months). Although continued monitoring is required to confirm these results and increase their accuracy, the present evidence indicates that luminous quasars have broad-line regions smaller than about 1 lt-year. Two of the quasars monitored show no detectable line variations despite relatively large-amplitude continuum changes. This could, however, be a stronger manifestation of the low-amplitude line-response phenomenon we observe in the other quasars.
