SDSS 1355+0856: A detached white dwarf + M star binary in the period gap discovered by the SWARMS survey

TL;DR

This paper reports the discovery and detailed characterization of a short-period detached white dwarf + M dwarf binary system in the period gap, providing insights into common envelope evolution and binary formation.

Contribution

It presents the first detailed analysis of SDSS 1355+0856, a rare short-period WD+M dwarf binary with a likely He-core white dwarf, enhancing understanding of binary evolution.

Findings

Period of 0.11438 days

Primary mass of 0.46 solar masses

Secondary mass between 0.083 and 0.097 solar masses

Abstract

SDSS 1355+0856 was identified as a hot white dwarf (WD) with a binary companion from time-resolved SDSS spectroscopy as part of the ongoing SWARMS survey. Follow-up observations with the ARC 3.5m telescope and the MMT revealed weak emission lines in the central cores of the Balmer absorption lines during some phases of the orbit, but no line emission during other phases. This can be explained if SDSS 1355+0856 is a detached WD+M dwarf binary similar to GD 448, where one of the hemispheres of the low-mass companion is irradiated by the proximity of the hot white dwarf. Based on the available data, we derive a period of 0.11438 +- 0.00006 days, a primary mass of 0.46 +- 0.01 solar masses, a secondary mass between 0.083 and 0.097 solar masses, and an inclination larger than 57 degrees. This makes SDSS 1355+0856 one of the shortest period post-common envelope WD+M dwarf binaries known, and one of only a few where the primary is likely a He-core white dwarf, which has interesting implications for our understanding of common envelope evolution and the phenomenology of cataclysmic variables. The short cooling time of the WD (25 Myr) implies that the system emerged from the common envelope phase with a period very similar to what we observe today, and was born in the period gap of cataclysmic variables.