The shortest period detached white dwarf + main-sequence binary

TL;DR

This paper reports detailed observations and system parameter measurements of the shortest period eclipsing white dwarf + main-sequence binary, revealing its likely helium core white dwarf nature and future evolution into a cataclysmic variable.

Contribution

It provides the first detailed characterization of a 94-minute white dwarf/main-sequence binary, including mass, radius, and evolutionary status, using high-speed photometry and spectroscopy.

Findings

White dwarf likely has a helium core with a mass near the upper limit from evolution.

Main-sequence star is an M8 dwarf close to the hydrogen burning limit.

System will evolve into a cataclysmic variable in ~400 million years.

Abstract

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94 minute orbital period eclipsing white dwarf / main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of 85.5 +/- 0.2 deg. The white dwarf has a mass of 0.51 +/- 0.05 Msun and a radius of 0.0247 +/- 0.0008 Rsun. With a temperature of 35,300 +/- 400K the white dwarf is highly over-inflated if it has a carbon-oxygen core, however if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 +/- 0.01 Msun and a radius of 0.110 +/- 0.004 Rsun placing it close to the hydrogen burning limit. The system emerged from a common envelope ~20 million years ago and will reach a semi-detached configuration in ~400 million years, becoming a cataclysmic variable with a period of 66 minutes, below the period minimum.