Long-term activity of GK Per (AAVSO data Kafka 2016). The one-day means are used. After the return to quiescence from the classical nova outburst (1901), the initial nova-like activity was followed by a series of DN outbursts. The points represent the one-day means and were connected by a line in the densely observed parts of the light curves to guide the eye. The vertical lines mark the borders of three segments (S1, S2, S3; see Sect. 3.3 for details).