Kawka, AdelaBriggs, GordonVennes, StephaneFerrario, LiliaPaunzen, ErnstWickramasinghe, Dayal2021-04-282021-04-280035-8711http://hdl.handle.net/1885/231069We 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 supernovaAK and SV acknowledge support from the Grant Agency of the Czech Republic (P209/12/0217 and 15-15943S) and the Ministry of Education of the Czech Republic (LG14013). This work was also supported by the project RVO:67985815 in the Czech Republic. SV acknowledges support from the Mathematical Sciences Institute of the Australian National University. EP acknowledges support by the Ministry of Education of the Czech Republic (grant LG15010). GPB gratefully acknowledges receipt of an Australian Postgraduate Award.application/pdfen-AU© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Societybinaries: closestars: individual: NLTT 12758stars: magnetic fieldwhite dwarfs201710.1093/mnras/stw31492020-11-23