  We study the soft X-ray variability of Cygnus X-3. By combining data from the
All-Sky Monitor and Proportional Counter Array instruments on the RXTE
satellite with EXOSAT Medium Energy (ME) detector observations, we are able to
analyse the power density spectrum (PDS) of the source from 10^(-9) - 0.1 Hz,
thus covering time-scales from seconds to years. As the data on the longer
time-scales are unevenly sampled, we combine traditional power spectral
techniques with simulations to analyse the variability in this range. The PDS
at higher frequencies (>10^-3 Hz) are for the first time compared for all
states of this source. We find that it is for all states well-described by a
power-law, with index ~-2 in the soft states and a tendency for a less steep
power-law in the hard state. At longer time-scales, we study the effect of the
state transitions on the PDS, and find that the variability below ~10^(-7) Hz
is dominated by the transitions. Furthermore, we find no correlation between
the length of a high/soft state episode and the time since the previous
high/soft state. On intermediate time-scales we find evidence for a break in
the PDS at time-scales of the order of the orbital period. This may be
interpreted as evidence for the existence of a tidal resonance in the accretion
disc around the compact object, and constraining the mass ratio to M2/M1<0.3.
