Grate 1 teaspoon of zest from the oranges. Peel the oranges with a sharp knife, removing all of the bitter white pith. Working over a bowl, cut in between the membranes to release the sections. Squeeze 1/4 cup of juice from the membranes into a small saucepan. Add the orange zest, vinegar, shallot and tarragon stems. Boil over high heat until reduced to 2 tablespoons, about 3 minutes. Remove from the heat and discard the tarragon stems. Whisk in the butter, 1 tablespoon at a time, until the sauce is creamy. Season with salt and pepper and stir in the chopped tarragon.