V476 Cyg (Nova Cyg 1920) is currently a dwarf nova -- first such an object in the period gap?

TL;DR

V476 Cyg, a classical nova from 1920, is now a dwarf nova with a potential orbital period in the period gap, offering insights into accretion disk behavior and nova evolution.

Contribution

This study reports the first identification of V476 Cyg as a dwarf nova within the period gap, with a potential orbital period of about 0.1018 days, and discusses its implications for nova evolution.

Findings

V476 Cyg is currently a dwarf nova with ~24-day cycle.

Possible orbital period of 0.1018 days places it in the period gap.

Transition to dwarf nova phase occurred ~100 years after eruption.

Abstract

V476 Cyg (Nova Cyg 1920) is a bright, fast nova reaching a photographic magnitude of 2.0. Using the Zwicky Transient Facility (ZTF) public database, I found that this nova is currently a dwarf nova with a cycle length of ~24 d. Compared to other classical novae currently in dwarf nova-type states, outbursts of V476 Cyg are rapidly rising and short with durations of a few days. Based on the AAVSO observations, this nova was probably already in the dwarf nova-type phase in 2016, 96 years after the nova eruption. I found a possible orbital period of 0.1018002(6) d using the ZTF data, which would place the object in the period gap. This supposed short orbital period appears to explain the features and faint absolute magnitudes of the observed dwarf nova outbursts. If this period is confirmed, V476 Cyg is a classical nova with the shortest orbital period with distinct dwarf nova outbursts and in which a nova eruption was recorded in the modern era. I also compared with the outburst properties with V446 Her (Nova Her 1960), which currently shows SS Cyg-type outbursts. The transition to the dwarf nova-phase in V476 Cyg occurred much earlier (~100 yr) than what has been supposed (~1000 yr) for classical novae below the period gap. V476 Cyg would not only provide an ideal laboratory of the behavior of an irradiated accretion disk in which tidal instability is expected to work, but also an ideal laboratory of the effect of a massive white dwarf on dwarf nova outbursts.