  We have for the first time measured the overall expansion rate of the
supernova remnant Cas A as seen in X-rays using ROSAT and Einstein HRI images
with time differences up to almost 17 years. The overall expansion timescale of
Cas A is found to be 501 plusminus 15 yr. This is significantly shorter than
the timescale based on high resolution radio data. Although the results clearly
indicate that Cas A is not anymore in the free expansion phase,the discrepancy
between the radio and X-ray timescales cannot be easily understood.
Furthermore, the expansion rate is incompatible with that predicted by a
self-similar model for a supernova remnant ploughing through the wind of its
progenitor.
