Series of Dwarf Nova Outbursts in the Post-novae AT Cancri and GK Persei

Abstract AT Cancri and GK Persei are post-novae (Nova Cnc in the seventeenth century and Nova Per 1901). Nowadays, they show episodic brightenings attributed to dwarf nova (DN) outbursts. We used CCD and visual observations from the AAVSO, CCD data from the Zwicky Transient Facility, and digitized photographic data from the Digital Access to a Sky Century @ Harvard database. We present the properties of the activities of these post-novae, which include, at least at times, a series of outbursts. The occurrence times of these events are not dramatically irregular. Their cycle lengths, T C , only display small epoch-to-epoch fluctuations, while considerable variations can occur on long timescales. This behavior of T C is similar to DNe, in which no classical nova (CN) eruption is known. We demonstrate that the energy emitted in individual outbursts in the optical band can differ substantially, even for neighboring outbursts of GK Per. At the same time, the length of T C varies on a longer timescale. Our findings have implications for understanding the behavior of T C . We find that the decaying branches of these outbursts’ light curves in AT Cnc are sometimes slower than in DNe of similar orbital period lengths that were not observed to erupt as a CN, and we attribute them to the so-called extra light in this system. The fraction of time AT Cnc spends in standstills decreases with time during observation, lasting more than 120 yr.