Rotation period plotted as a function of the activity-cycle period. Depicted as blue and green triangles are stars belonging to inactive and active sequence (Böhm-Vitense 2007). Represented as a magenta circles are stars with X-ray cycles (cf. see introduction). The place of AB Dor A is represented as magenta star. Depicted as red circles are the activity cycles reported in other ultra-fast rotators reported by Bernhard & Frank (2006), Taš (2011), and Vida et al. (2013).