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 ( T _ { \mathrm { eff } } \simeq 6300 K , \log g = 5 to 6.6 ) with broad wings from a hotter , high-mass white dwarf companion ( 11 , 000 to 14 , 000 K ; \sim 1 \mbox { M } _ { \odot } ) .
The high-mass white dwarf has unusual line profiles which lack the narrow central core to H \alpha that is usually seen in white dwarfs .
This is consistent with rapid rotation with v \sin i = 500 to 1750 \mbox { km } \mbox { s } ^ { -1 } , although other broadening mechanisms such as magnetic fields , pulsations or a helium-rich atmosphere could also be contributory factors .
The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature .
Within the constraints set by our data , SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor .
However , SDSSJ1257+5428 ’ s unusually low mass ratio q \approx 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf ( AM CVn star ) .