Brown dwarves are candidates to a potential sun's dual
Brown dwarf stars are stars that have too little mass to fuse hydrogen (although deuterium is likely to fuse for a brief period). Interestingly enough, they are all about the size of Jupiter, regardless of their mass. Brown dwarfs are heated as they slowly collapse by conversion of their gravitational potential energy into thermal energy. The upper mass limit for a star to be a brown dwarf is about 0.07-0.08 solar masses, while the transition to being considered a large planet takes place by convention at 0.012 solar masses, where even deuterium fusion stops.
A binary system (WISE J1049-5319 A and B made up of 2 brown dwarves was recently discovered pretty close to the solar system. Alpha centauri, the closest knwon to the sun, is a ternary system.
