The Zilpah reference article from the English Wikipedia on 24-Apr-2004
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Zilpah

In the Book of Genesis, Zilpah is a concubine of Jacob and the mother of Gad and Asher.

She was a slave of Laban, and was given to his daughter Leah as handmaiden on the occasion of her marriage to Jacob, see Genesis 29:24, 46:18. Leah later presented her to her husband Jacob as a concubine.