Castor and Pollux (or Polydeuces) are 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 raped Leda in the guise of a swan. The pair are thus an example of heteropaternal superfecundation. 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 (literally "twins") 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 (11.298–304), they are the sons of Tyndareus alone, but they were sons of Zeus in the Hesiodic Catalogue (fr. 24 M–W). The conventional account (attested first in Pindar, Nemean 10) 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 name Tindaridai in Spartan inscriptions, or Tyndaridai in literature, in turn occasioning incompatible accounts of their parentage. Their other sisters were Timandra, Phoebe, and Philonoe.
Castor and Pollux are sometimes both mortal, sometimes both divine. One consistent point is that if only one of them is immortal, it is Pollux. In Homer's Iliad, Helen looks down from the walls of Troy and wonders why she does not see her brothers among the Achaeans.