# Optical Spectroscopy of the Most Compact Accreting Binary Harboring a Magnetic White Dwarf and a Hydrogen-rich Donor

## TL;DR

This paper reports the discovery and analysis of Gaia19bxc, a unique magnetic white dwarf binary with an exceptionally short orbital period of 64.42 minutes, likely being a metal-poor system that challenges existing models of cataclysmic variables.

## Contribution

It presents the first detailed optical spectroscopic and photometric study of a likely metal-poor polar with an ultra-short orbital period below the canonical minimum.

## Key findings

- Gaia19bxc has an orbital period of 64.42 minutes.
- The system likely contains a metal-poor, hydrogen-rich donor.
- Gaia19bxc is the shortest-period magnetic CV known.

## Abstract

Accreting white dwarfs in close binary systems, commonly known as cataclysmic variables (CVs), with orbital periods below the canonical period minimum ($\approx$ 80 minutes) are rare. Such short periods can only be reached if the donor star in the CV is either significantly evolved before initiating mass transfer to the white dwarf (WD) or metal-poor. We present optical photometry and spectroscopy of Gaia19bxc, a high-amplitude variable identified as a polar CV with an exceptionally short orbital period of 64.42 minutes - well below the canonical CV period minimum. High-speed photometry confirms persistent double-peaked variability consistent with cyclotron beaming, thus indicating the presence of a magnetic WD. Phase-resolved Keck/LRIS spectroscopy reveals strong hydrogen and helium emission lines but no donor features, indicating the accretor is a magnetic WD and the donor is hydrogen-rich, but cold and faint. The absence of a detectable donor and the low inferred temperature ($\lesssim$ 3500 K) disfavor an evolved donor scenario. Instead, the short period and the system's halo-like kinematics suggest Gaia19bxc may be the first known metal-poor polar. Because metal-poor donors are more compact than solar-metallicity donors of the same mass, they can reach shorter minimum periods. Gaia19bxc is one of only a handful of known metal-poor CVs below the canonical period minimum and has the shortest period of any such magnetic system discovered to date.