Five Seyfert 1 galaxies of high absolute luminosity (ranging from 28 to 65 times that of NGC4151 at 3.5 μm) have been monitored since 1984 (or earlier) at JHK & L (1.25, 1.65, 2.2 & 3.5 μm). The results are shown in the accompanying figures. Where the variations were of sufficient amplitude, changes in L are seen to have been delayed ∼ 500 days compared to those at shorter wavelengths. In all cases, even in 3A 0557-383 where the amplitude of variation is small, each galaxy traces out loops with

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... time if magnitudes at two different wavelengths are plotted against each other. The loops are always described in the same sense (clockwise as plotted here), such that variations at the longer wavelengths are seen to be retarded compared to those at the shorter. Plots of this kind appear to be a sensitive way to detect delays even when the data are too sparse to support a credible cross-correlation.