# A 9-Hr CV With One Outburst in 4 Years of Kepler Data

## TL;DR

This paper reports the discovery and detailed analysis of a unique cataclysmic variable star with only one outburst in four years, providing insights into its system parameters, evolution, and potential as a progenitor of AM CVn binaries.

## Contribution

The study presents the first detailed characterization of a CV with a single outburst over four years, including system masses, inclination, and evolutionary modeling, highlighting its significance as a progenitor of AM CVn systems.

## Key findings

- The white dwarf mass is the lowest reported for an accreting WD in a CV.
- The system is likely to exhibit dwarf nova outbursts despite near-critical mass transfer rates.
- This binary is a probable progenitor of AM CVn binaries evolved through the CV channel.

## Abstract

During a visual search through the Kepler main-field lightcurves, we have discovered a cataclysmic variable (CV) that experienced only a single 4-day long outburst over four years, rising to three times the quiescent flux. During the four years of non-outburst data the Kepler photometry of KIC 5608384 exhibits ellipsoidal light variations (`ELV') with a $\sim$12% amplitude and period of 8.7 hours. Follow-up ground-based spectral observations have yielded a high-quality radial velocity curve and the associated mass function. Additionally, H$\alpha$ emission lines were present in the spectra even though these were taken while the source was presumably in quiescence. These emission lines are at least partially eclipsed by the companion K star. We utilize the available constraints of the mass function, the ELV amplitude, Roche-lobe filling condition, and inferred radius of the K star to derive the system masses and orbital inclination angle: $M_{\rm wd} \simeq 0.46 \pm 0.02 \, M_\odot$, $M_{\rm K} \simeq 0.41 \pm 0.03 \, M_\odot$, and $i \gtrsim 70^\circ$. The value of $M_{\rm wd}$ is the lowest reported for any accreting WD in a cataclysmic variable. We have also run binary evolution models using MESA to infer the most likely parameters of the pre-cataclysmic binary. Using the mass-transfer rates from the model evolution tracks we conclude that although the rates are close to the critical value for accretion disk stability, we expect KIC 5608384 to exhibit dwarf nova outbursts. We also conclude that the accreting white dwarf most likely descended from a hot subdwarf and, most notably, that this binary is one of the first bona fide examples of a progenitor of AM CVn binaries to have evolved through the CV channel.