The Gudrun reference article from the English Wikipedia on 24-Jul-2004
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Gudrun

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In Norse mythology, Gudrun was the sister of Gunnar, who failed in rescuing the Valkyrie Brünnehilde. Sigurd then succeeded in doing so, taking the shape of Gunnar. After marrying Brünnehilde, Sigurd left her for Gudrun (because the witch Grimhild had cast a spell on him).