Castor and Pollux/Dioskouroi. Castor and Pollux were twin half-brothers in Greek and Roman mythology, known together as the Dioscuri.
   Their mother was Leda, but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine son of Zeus, who seduced Leda in the guise of a swan. Though accounts of their birth are varied, they are sometimes said to have been born from an egg, along with their twin sisters Helen of Troy and Clytemnestra.
   In Latin the twins are also known as the Gemini or Castores, as well as the Tyndaridae or Tyndarids. Pollux asked Zeus to let him share his own immortality with his twin to keep them together, and they were transformed into the constellation Gemini.
   The pair were regarded as the patrons of sailors, to whom they appeared as St. Elmo's fire. They were also associated with horsemanship, in keeping with their origin as the Indo-European horse twins.
   There is much contradictory information regarding the parentage of the Dioscuri. In the Homeric Odyssey, they are the sons of Tyndareus alone, but they were sons of Zeus in the Hesiodic Catalogue. The conventional account combined these paternities so that only Pollux was fathered by Zeus, while Leda and her husband Tyndareus conceived Castor. This explains why they were granted an alternate immortality. The figure of Tyndareus may have entered their tradition to explain their archaic
Wikipedia ...