| Summary: | We present an analysis of the close double degenerate NLTT 12758, which is comprised of
a magnetic white dwarf with a field of about 3.1 MG and an apparently non-magnetic white
dwarf. We measured an orbital period of 1.154 d and found that the magnetic white dwarf is
spinning around its axis with a period of 23 min. An analysis of the atmospheric parameters
has revealed that the cooling ages of the two white dwarfs are comparable, suggesting that
they formed within a short period of time from each other. Our modelling indicates that the
non-magnetic white dwarf is more massive (M = 0.83 M) than its magnetic companion
(M = 0.69 M) and that the total mass of the system is higher than the Chandrasekhar mass.
Although the stars will not come into contact over a Hubble time, when they do come into
contact, dynamically unstable mass transfer will take place leading to either an accretion
induced collapse into a rapidly spinning neutron star or a Type Ia supernova.
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