LEDA

One Version:   Leda was married to Tyndareus, king of Sparta.  Zeus came to her in the form of a swan, and she produced four children from two eggs.  From one sprang Pollux and Helen, and from the other Castor and Clytemnestra.  Castor and Pollux journeyed with Jason as part of the crew of the Argo on the quest for the Golden Fleece.   Clytemnestra and Helen played and important role in the story of the Trojan war and its aftermath.

Another Version:   Another one of the many loves of Zeus was Leda, the daughter of Thestius, King of Aetolia who bore the god two sets of twins after he came to her in the form of a swan.  One set of twins, Castor and Clytemnestra, were mortal, while the other set, Polux and Helen, were immortal.