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arXiv · 2610.19105

A Distinct Type of Standstill in Ultra-Compact White Dwarf Binaries

Jan Kára·Liliana Rivera Sandoval·Iwona Kotko·Jean-Marie Hameury·Luis E. Salazar Manzano·Ryan J. Oelkers
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Abstract

AM CVn stars are ultra-compact, semi-detached binaries consisting of a white dwarf primary and a hydrogen-poor secondary. In this work, we analyse the AM CVn-type systems MGAB-V270, ZTF18aaxuusk and ASASSN-19ct, which have similar properties and show unusual post-superoutburst behaviour. We focus on the occurrence of standstill-like phases. In MGAB-V270, superoutbursts are followed by a series of echo outbursts that evolves into a standstill-like phase before the system returns to quiescence. ZTF18aaxuus shows similar post-superoutburst activity, with one echo-outburst series also developing into a standstill-like phase. ASASSN-19ct exhibits comparable echo outbursts but no standstill-like phases. Periodicity analysis using TESS data shows that superhumps can persist during both echo outbursts and standstill-like phases. Although standstills have previously been observed in AM CVn stars, the standstill-like phases we observe appear to represent a distinct phenomenon associated with post-superoutburst activity. We compare our results to other AM CVns that may exhibit similar behaviour.

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Jan Kára, Liliana Rivera Sandoval, Iwona Kotko, Jean-Marie Hameury, Luis E. Salazar Manzano, Ryan J. Oelkers. 2026-10-13. A Distinct Type of Standstill in Ultra-Compact White Dwarf Binaries. https://arxiv.org/abs/2610.19105

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