Measurement of the broad-line region size in two bright quasars

We present 4 years of spectrophotometric monitoring data for two radio-quiet quasars, PG 0804+762 and PG 0953+414, with typical sampling intervals of several months. Both sources show continuum and emission-line variations. Tne variations of the H beta line follow those of the continuum with a time lag, as derived from a cross-correlation analysis, of 93+/-30 days for PG 0804+762 and 111+/-55 days for PG 0953+414. This is the first reliable measurement of such a lag in active galactic nuclei with luminosity L > 10(45) ergs s(-1). The broad-line region (BLR) size that is implied is almost an order of magnitude larger than that measured in several Seyfert 1 galaxies and is consistent with the hypothesis that the BLR size grows as L(0.5).