  We report on some results from a three-year program of RXTE observations of 4
Seyfert galaxies: NGC 4051, NGC 5548, MCG-6-30-15 and NGC 5506. We focus here
on color-color and color-flux diagrams obtained using the count-rates of the
PCA instrument in different energy ranges: 3--5, 5--7 and 7--10 keV. The data
of the different sources show interesting trends in the different color-color
and color-flux planes. These trends are quite similar from one source to the
other, even in the case of the narrow line Seyfert 1 NGC 4051 which exhibits
strong flux and spectral variability. We discuss these results in term of a
simple spectral model: a power law with high energy cut-off + reflection + Iron
line, with the scope of understanding the relations between the observed
variations of different components and the physical causes of the variability.

Astrophysics