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

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.

This version cites a recent paper about Gaia19bxc (Galiullin et al. 2025, ApJL, 990, L57) that C.L. had missed in his literature review. The discussion in the appendix has been updated accordingly, with relevant changes described in footnote 5. C.L. is grateful to Vladislav Dodon for alerting him to this oversight