# Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

## TL;DR

This paper reports the discovery of two ultrashort-period eclipsing binary systems containing white dwarfs and likely substellar companions, including a brown dwarf, using Kepler K2 data, with follow-up observations to estimate their physical parameters.

## Contribution

First identification of two ultrashort-period detached white dwarf binaries with substellar companions using K2 data, including detailed follow-up and preliminary parameter estimates.

## Key findings

- One system has a brown dwarf companion of 0.049 M_sun.
- The other system's companion is likely a brown dwarf or low-mass star.
- Both systems have orbital periods around 71-72 minutes.

## Abstract

Using data from the extended Kepler mission in K2 Campaign 10 we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205-0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf, and the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205-0242 is composed of a $0.39\pm0.02$M$_\odot$ helium-core white dwarf which is totally eclipsed by a $0.049\pm0.006$M$_\odot$ ($51\pm6$M$_J$) brown dwarf companion, while SDSS J1231+0041 is composed of a $0.56\pm0.07$M$_\odot$ white dwarf which is partially eclipsed by a companion of mass $\lesssim 0.095$M$_\odot$. In the case of SDSS J1205-0242 we look at the combined constraints from common-envelope evolution and brown dwarf models; the system is compatible with similar constraints from other post common-envelope binaries given the current parameter uncertainties, but has potential for future refinement.