Having been so unfortunate in their wives, the two heroes determined not to seek again any king's daughters in marriage, but to choose some one of divine parentage. So Theseus decided upon Helen, the child of Jupiter and Leda[105] whom the ruler of the gods wooed in the form of a snow-white swan. Pirithous's choice fell upon Proserpina, the wife of Pluto. Theseus succeeded in carrying off Helen; but as she was at that time a mere child, he left her in the care of his mother Æthra until she should be old enough for marriage. Meanwhile Helen's twin brothers, Castor and Pollux, having discovered who her abductor was, waited until he was absent on the venture with his friend Pirithous, and then went to the rescue of their sister, whom they took forcibly from Æthra, and carried triumphantly home to Sparta. Of these twin brothers, Castor was mortal, and some time later was slain in battle. Pollux, who was immortal, then begged Jupiter to let him die also, that he might not be separated from his brother. The ruler of Olympus, touched by this evidence of devotion, allowed Castor to return to life on condition that Pollux would spend half of his time in Hades. Later on both brothers were translated to the heavens, where they form a bright constellation, one of the signs of the Zodiac.[106] Although Theseus's attempt to win an immortal bride was unsuccessful, the two friends were not discouraged; and the hero accompanied Pirithous to Hades when he made the bold attempt to carry off Proserpina. Pluto, having discovered their intention, was so enraged at the insult that he fastened Theseus to an enchanted rock, and bound Pirithous to the ever-revolving wheel of his father Ixion. When Hercules went down to Hades to ask Pluto's permission to carry the three-headed dog Cerberus to the court of Eurystheus,[107] he freed Theseus from the enchanted rock and thus enabled him to return to Athens.