Trim ends from beans. Heat olive oil in a large sauté pan over medium high heat. Add beans and toss to coat. Add garlic and orange zest and sauté 1 minute. Add 2/3 cup water and simmer beans uncovered until almost all the water has evaporated, about 4 minutes. Remove beans from pan onto serving dish. Note that haricots verts will cook faster than regular green beans.
Add orange and grapefruit segments and stir gently to warm through – juices will come out. When juices begin to simmer, stir in sugar and remove from heat. Stir in cold butter in small pieces, until melted. Season to taste and spoon over beans.