Where is the closest binary star system?
Alpha Centauri A & B
Our nearest star system Alpha Centauri includes a binary star system. Alpha Centauri A is slightly bigger than the Sun and Alpha Centauri B is slightly smaller than the Sun. These form a binary system which is about 4.37 light years away.
There is a third star associated with the system called Alpha Centauri C or Proxima Centauri which is the closest star which is external to our solar syatem.