SIMBAD references

We present an analysis of EXOSAT ME X-ray colour-colour diagrams of Cygnus X-3. Our sample contains all the Cygnus X-3 data collected during EXOSAT's operational life time and covers a total of 190 hours. The colour-colour diagrams display X-ray spectral variations with the 4.8 hour period and show the spectral distinction between the `high' and the `low' intensity state. We find that the 4.8 hour spectral variability produces a `loop' in the colour-colour diagram. When the 4.8 hour effect is removed, the spectra display residual long-term variability that is suggestive of the presence of `branches' in the colour-colour diagram.