In the Canon Comic Book by Disney's Marvel Star Wars issue #6, we learn how Darth Vader found out he even had a son, from Boba Fett. Boba was sent to track down Luke on Tatooine after the events of A New Hope, in order to bring him before Vader, alive. Failing to do so, Boba returns to Vader with nothing other than Luke's last name, Skywalker.

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