Detection of a white dwarf companion to the white dwarf SDSSJ125733.63+542850.5
T.R. Marsh, B.T. Gaensicke, D. Steeghs, J. Southworth, D. Koester, V., Harris, L. Merry

TL;DR
This paper confirms the presence of a second white dwarf in the binary system SDSSJ1257+5428 through spectroscopy and ultraviolet data, revealing a complex system with potential supernova progenitor characteristics.
Contribution
It provides the first direct detection and characterization of the second white dwarf in SDSSJ1257+5428, including its temperature, gravity, and possible rapid rotation.
Findings
The system contains a cool, low gravity white dwarf and a hotter, high-mass white dwarf.
The high-mass white dwarf shows unusual broad line profiles possibly due to rapid rotation.
SDSSJ1257+5428 may evolve into a double white dwarf system rather than a supernova progenitor.
Abstract
SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hours. While an initial analysis suggested the presence of a neutron star or black-hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low gravity white dwarf (Teff ~6300K, log g = 5 to 6.6) with broad wings from a hotter, high-mass white dwarf companion (11,000 to 14,000K; ~ 1Msun). The high-mass white dwarf has unusual line profiles which lack the narrow central core to Halpha that is usually…
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