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-circle dot and a more massive companion of similar to 1 M-circle dot that is more than 0.6 Gyr younger. The origin of this system has been termed para