The fraternal twins Castor and Pollux are the sons of Leda (the wife of a mortal named Tyndareus). Zeus had disguised himself as a swan and seduced Leda, who laid an egg from which Helen and Pollux were born. At the same time she also gave birth to Castor and Clytaemnestra, children of her husband Tyndareus. Since Pollux was the son of a Zeus, he was immortal, but Castor was mortal. The brothers grew close. At the Olympic games Castor was killed. Pollux asked Zeus to permit him to die so they could remain together. Zeus placed both of the boys in the sky.