TESS light curves of two new magnetic cataclysmic variables: an asynchronous polar at the period minimum, and an eclipsing system with a large spin-to-orbit ratio

TL;DR

This paper reports the discovery of two new magnetic cataclysmic variables with short orbital periods and high spin-to-orbit ratios, supporting theories of their synchronization evolution.

Contribution

It introduces two new magnetic cataclysmic variables, including a candidate asynchronous polar at the period minimum, expanding understanding of their properties and evolution.

Findings

Gaia21akb has a probable orbital period of 1.29 h, second-shortest known polar.

ZTF18aazmehw is an eclipsing system with a 1.50 h period and high spin-to-orbit ratio.

ZTF18aazmehw shows no evidence of pole switching, possibly having a disk-like structure.

Abstract

A recent development in the study of magnetic cataclysmic variable stars (mCVs) has been the identification of asynchronously spinning mCVs with orbital periods <2 h that have significantly higher white dwarf spin-to-orbital period ratios than their longer-period counterparts. We report the discovery of two additional mCVs in this class. The first, Gaia21akb, is a candidate asynchronous polar at the period minimum. While TESS photometry cannot, in isolation, lead to a conclusive identification of the orbital period, the probable orbital period of 1.29 h would be the second-shortest of any known polar and would result in a spin-to-orbit ratio of 0.9879. The second system in our study, ZTF18aazmehw, is an eclipsing mCV with a 1.50 h orbital period and a spin-to-orbit ratio of 0.867. Contrary to expectations for an asynchronous polar, ZTF18aazmehw does not show discernible evidence of pole switching and might possess a disk-like structure. The increasing number of short-period asynchronous mCVs with large spin-to-orbit ratios lends credence to theoretical predictions that asynchronously rotating mCVs with sufficiently strong white dwarf magnetic fields can achieve synchronization when their orbital separations have shrunk sufficiently.