The twins were born as mortal princes, sons of the Spartan queen Leda, one being fathered by Zeus the other by her husband Tyndareus. Because of their generosity and kindness to man they were apotheosed into gods at death. At first Polydeukes alone, being a son of Zeus, was offered this gift, but he agreed only on condition that his half-twin Kastor share the honor.
Milne 1403v Hadrian
Milne (1982) plate II coin 1403v
Curtis (1990) p. 287 coin 419