In a large soup pot, melt one stick of butter. Add the celery, onions, white pepper and cayenne. Stir and cook over low heat until vegetables begin to soften. Over low heat in another pan, make a roux by combining 8 tablespoons melted butter and the flour to make a base for thickening the soup. Cook at least 2 minutes, stirring constantly, so the floury taste is eliminated. Add the roux and the cheese to the soup pot. Add the water, cream, oysters and their liquid. Simmer the soup until the oysters begin to curl. Add the champagne and sherry and heat through.