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We present results from RXTE data obtained during a systematic monitoringprogram of four Seyfert galaxies (NGC5548, NGC5506, MCG-6-30-15 and NGC4051).We studied the variability of three hardness ratios derived from the lightcurves in four energy bands. All the objects show similar spectral variationsin all ratios. In order to interpret the results we computed the hardnessratios corresponding to a simple spectral model of a power law plus iron lineplus reflection component, considering two possibilities: a)variations withconstant line equivalent width and reflection parameter R and b) variationswith constant line and reflection flux. The overall, mean observed trends canbe explained by spectral slope variations and a constant flux Fe line andreflection component, although the existence of a line component which isvariable on short time scales cannot be excluded. Finally, we find that thedata are not consistent with an increase of R with flux for individual sources,indicating that, as a single source varies, softer spectra do not correspond tolarger R values.Comment: 24 pages, 8 figures, accepted for publication in Ap

Long‐Term Spectral Variability of Seyfert Galaxies fromRossi X‐Ray Timing ExplorerColor‐Flux Diagrams