Heat the butter in a medium-size heavy saucepan over medium heat. Add the beans and pear and toss to coat. Stir in the wine and water. Cover partially and cook over low heat until the pear is very tender, 4 to 5 minutes. Remove the cover and season with the salt, pepper, and tarragon. Stir in the cream. Cook over high heat until the cream begins to evaporate and the sauce thickens and begins to coat the beans, about 4 minutes. The pears will melt into the sauce and cling to the cooked beans. Serve very hot.