  Using the archival ROSAT PSPC observations, AB Dor is found to be variable in
X-rays. The periodic variations are consistent with previously reported
rotational period of 0.514 days. The average spectrum of AB Dor is best
represented with two-temperature Raymond-Smith model with kT values of
0.19+/-0.07 and 1.17+/-0.02 keV. The quiescent luminosity of the system is
found to be 4.36+/-0.6 10^30 ergs/s. A flare with a rise time of about 350
seconds is detected during which X-ray luminosity rises from 5.8+/-1.6 10^30 to
15.8+/-4.9 10^30 ergs/s. We conclude that AB Dor is very similar to the active
components of RS CVn binaries and other active classes. In view of the wide
separation from the binary companion Rst 137B, this activity must be intrinsic
to the active star.
