Context : Although chromospheric activity cycles have been studied in a larger number of late-type stars for quite some time , very little is known about coronal activity-cycles in other stars and their similarities or dissimilarities with the solar activity cycle .

Aims : While it is usually assumed that cyclic activity is present only in stars of low to moderate activity , we investigate whether the ultra-fast rotator AB Dor , a K dwarf exhibiting signs of substantial magnetic activity in essentially all wavelength bands , exhibits an X-ray activity cycle in analogy to its photospheric activity cycle of about 17 years and possible correlations between these bands .

Methods : We analysed the combined optical photometric data of AB Dor A , which span \sim 35 years .
Additionally , we used ROSAT and XMM-Newton X-ray observations of AB Dor A to study the long-term evolution of magnetic activity in this active K dwarf over nearly three decades and searched for X-ray activity cycles and related photometric brightness changes .

Results : AB Dor A exhibits photometric brightness variations ranging between 6.75 ~ { } < ~ { } V _ { mag } ~ { } \leq~ { } 7.15 while the X-ray luminosities range between 29.8 ~ { } < ~ { } log~ { } L _ { X } ~ { } [ erg / s ] ~ { } \leq~ { } 30.2 in the 0.3-2.5 keV .
As a very active star , AB Dor A shows frequent X-ray flaring , but , in the long XMM-Newton observations a kind of basal state is attained very often .
This basal state probably varies with the photospheric activity-cycle of AB Dor A which has a period of \sim 17 years , but , the X-ray variability amounts at most to a factor of \sim 2 , which is , much lower than the typical cycle amplitudes found on the Sun .

Conclusions :