Preheat oven to 400 degrees F.
Bake the sweet potatoes in the middle of the oven for 45 minutes to 1 hour, or until tender. Let them cool. Peel the sweet potatoes and cut them diagonally into 1/4-inch slices, and place them in a buttered 14-inch gratin dish. In a stainless steel or enameled saucepan, cook the remaining ingredients over moderate heat, stirring, until the sugar is dissolved. Spoon the mixture over the sweet potatoes and bake the gratin in the middle of the oven, basting occasionally, for 30 minutes.
The uncooked gratin, can be assembled 1 day ahead and kept refrigerated, and covered. Put the gratin under a preheated broiler about 4 inches from the heat until the edges are browned lightly.