The close magnetic/nonmagnetic double-degenerate binary LB 11146

Spacecraft imaging of the remarkable strongly magnetic/nonmagnetic white dwarf binary system LB 11146 establishes an upper limit to the component separation of 0″.025, corresponding to a 90% likelihood that the orbital separation is a < 500 R_⊙. The large and nearly identical inferred masses for the two components make it very likely that some amount of material was exchanged in the past; however, a true common envelope is not required. It is difficult to conceive of scenarios that could have produced the current combination of remnants from initially very different component masses, suggesting that the production of this (and other) strongly magnetic/nonmagnetic pairs may be more a result of unusual initial conditions in the protostellar cloud than of ensuing stellar evolution. While a mass merger will apparently not occur on a timescale relevant to the production of Type I supernovae, the super-Chandrasekhar total mass of LB 11146 and its relatively short orbital period suggest that binary evolution can produce the requisite progenitors.