A nice Double- Double star system to observe is Epsilon Lyrae, very close to Vega (one of the stars of the summer triangle)
Image credit : Torsten Bronger CC BY-SA 3.0 Published in seasky.org
Look for this in your finderscope or with a pair of binoculars and you will see a pair of stars, but this pair are special. On a clear night and with a 3inch or larger scope you will be able to split each star in this pair. The Double- Double is a complex multiple star system, a collection of stars 200 light years away. The pair to the North 1A and 1B lie about 150AU apart from each other, and take over 1,000 years to orbit each other; the pair to the South, 2A and 2B, are also about 150 AU apart, but they take about 600 years to orbit about there centre of mass. This pair orbits more quickly because they are more massive. The two pairs also orbit each other and are separated by about 0.2 light years and must take something like half a million years to complete one orbit about their common centre of mass.