Serve this cocktail with salted toasted almonds or a rich, garlicky cured sausage such as saucisson sec.
Gently muddle basil and orange with a wooden spoon or muddler until basil is barely bruised but fragrant. Stir in wine, and let sit at least 15 minutes and up to 2 hours.
Divide among 6 ice-filled glasses. Top each with 2 ounces seltzer, and serve immediately.