MGAB-V859 and ZTF18abgjsdg: ER UMa-type dwarf novae showing standstills

TL;DR

This study identifies two ER UMa-type dwarf novae, MGAB-V859 and ZTF18abgjsdg, that exhibit both ER UMa-type and Z Cam-type states, providing evidence of their transitional nature between dwarf nova and novalike states.

Contribution

First detailed observational evidence of dwarf novae transitioning between ER UMa-type and Z Cam-type states, supporting theories of their hybrid behavior.

Findings

MGAB-V859 showed transitions from ER UMa to standstill and Z Cam states.

ZTF18abgjsdg exhibited standstills during its typical ER UMa behavior.

Supercycles during ER UMa phases were approximately 55-58 days.

Abstract

Using Public Data Release of Zwicky Transient Facility observations, we found that MGAB-V859 and ZTF18abgjsdg are dwarf novae and show both ER UMa-type and Z Cam-type states. There had been only two dwarf novae showing similar transitions between the ER UMa-type and Z Cam-type states. MGAB-V859 showed both a transition from the ER UMa-type state to a long standstill in 2019 and a Z Cam-type state in 2020. During the standstill in 2020, this object faded twice and showed dwarf nova-type variations. ZTF18abgjsdg usually showed ER UMa-type behavior but standstills were seen in 2018 and 2019. The supercycles of these objects during the typical ER UMa-type phase were 55 d and 58 d, respectively. These objects provide additional evidence that some ER UMa stars indeed bridge between dwarf nova and novalike states as proposed in Kato et al. (2016).