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

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.