Superhumps in Cataclysmic Binaries. XV. EG Cancri, King of the Echo Outbursts

Abstract

ABSTRACTWe report photometry and spectroscopy of the dwarf nova EG Cancri in its 1996/1997 episode of eruptions. The main eruption was clearly a superoutburst featuring common superhumps with a period of 0.06036(2) days, establishing the star as a new member of the SU UMa class of dwarf novae. At the end of the main eruption, the superhumps were replaced by another wave of slightly longer period (0.06045 days), which endured for at least another 90 days. The properties of the latter wave suggest identification as a “late superhump.” The recurrence time for superoutbursts is long, probably in the range 7–20 yr. After the 15 day superoutburst, the star displayed six remarkable surges in light, with an average interval of 7 days; these “echo outbursts” strongly resemble ordinary dwarf nova eruptions. This suggests that a high accretion rate persisted in the disk for ∼40 days after the main superoutburst. By a year after outburst, the star faded to V = 18.7, with most of the continuum light from a white dwarf of T∼15,000 K. We estimate a distance of 320 pc and a 〈Mv〉 = +11.5 for accretion light. The extreme faintness, the rarity of eruptions, the enormous duration of superhumps, and the quiescent spectrum together establish membership in the WZ Sge subclass, the most sluggish of dwarf novae. Spectroscopy at quiescence and a transient wave in early outburst establish an orbital period of 0.05997(9) days. This suggests a secondary star with M∼0.02 M⊙, yet R∼0.08 R⊙. A binary can reach such a state after the secondary is forced to lose mass on a timescale shorter than its Kelvin‐Helmholtz timescale.