  We present results from RXTE data obtained during a systematic monitoring
program of four Seyfert galaxies (NGC5548, NGC5506, MCG-6-30-15 and NGC4051).
We studied the variability of three hardness ratios derived from the light
curves in four energy bands. All the objects show similar spectral variations
in all ratios. In order to interpret the results we computed the hardness
ratios corresponding to a simple spectral model of a power law plus iron line
plus reflection component, considering two possibilities: a)variations with
constant line equivalent width and reflection parameter R and b) variations
with constant line and reflection flux. The overall, mean observed trends can
be explained by spectral slope variations and a constant flux Fe line and
reflection component, although the existence of a line component which is
variable on short time scales cannot be excluded. Finally, we find that the
data 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 to
larger R values.
