Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

TL;DR

This study uses high-speed photometry of 20 short-period white dwarf binaries to discover eclipses, tidal distortions, and Doppler beaming, providing empirical constraints on low-mass white dwarf properties and gravitational wave sources.

Contribution

First identification of eight tidally distorted white dwarfs, including four new discoveries, and empirical constraints on the mass-radius relation for extremely low-mass white dwarfs.

Findings

Discovered 8 tidally distorted white dwarfs, 4 new.

Detected Doppler beaming confirming high radial-velocity variability.

All systems are potential gravitational wave sources.

Abstract

We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.