Panel a: times of the observed outbursts in X Ser, marked in Fig. 3e,g. The horizontal line marksthe times of the outbursts (solid circles). The associated open circles denote the time between the specific outburst (marked by the solid circle) and the time elapsing to the subsequent observed outburst. Panel b: histogram of the times between the neighboring observed outbursts (see Sect. 3.2 for details).