Letter to the Editor

The third known triple white dwarf: The close double white dwarf SDSS J125733.63+542850.5 hosts a white dwarf tertiary

¹ Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso, Chile
² Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
³ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁵ Max-Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
⁶ Department of Physics, University of Warwick, Gibbet Hill, Coventry CV4 7AL, UK
⁷ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany

^⋆ Corresponding author; matthias.schreiber@usm.cl

Received: 21 November 2024
Accepted: 10 December 2024

Abstract

The white dwarf (WD) binary SDSS J1257+5428 comprises an extremely low-mass WD with a mass of 0.1–0.24 M_⊙ and a more massive companion of ∼1 M_⊙ that is more than 0.6 Gyr younger. The origin of this system has been termed paradoxical because current theories for the formation of WD binaries are unable to explain its existence. Any additional observational constraint on the formation of SDSS J1257+5428 might provide important insights into double WD formation in general. We present the discovery of a tertiary WD, which makes SDSS J1257+5428 the third known triple WD. We used KECK/LRIS spectroscopy, Gaia and SDSS photometry, and WD atmosphere models to characterize the distant tertiary (projected separation ∼8000 au). We find the tertiary WD to be cool (6200 − 6400 K) and massive (log(g) = 8.88 − 8.97), which translates to a cooling age of ≳4 Gyr. This cooling age represents a lower limit on the total age of the triple system. While at first glance it seems likely that the inner binary formed through a stable mass transfer phase followed by common envelope evolution, reproducing the stellar mass and period required for the progenitor of the ∼1 M_⊙ WD in the inner binary through stable mass transfer seems impossible. We therefore speculate that the system might be the descendant of a cataclysmic variable with an evolved donor.