Preparation Combine prunes and Armagnac in an airtight container and let macerate at room temperature at least 24 hours. Bring sugar and 3/4 cup cold water to a boil in a 1-quart heavy saucepan, stirring until sugar is dissolved, then reduce heat and simmer 2 minutes. Purée prune and Armagnac mixture with sugar syrup in a food processor until very smooth, about 2 minutes. Add orange juice and remaining 1 1/4 cups cold water and pulse to combine. Force through a fine-mesh sieve into a bowl and discard solids. Chill until cold, about 1 hour. Freeze in ice cream maker, then transfer to an airtight container and put in freezer to harden, at least 2 hours. Cooks' note:Sorbet can be made 1 day ahead.
Preparation Combine prunes and Armagnac in an airtight container and let macerate at room temperature at least 24 hours. Bring sugar and 3/4 cup cold water to a boil in a 1-quart heavy saucepan, stirring until sugar is dissolved, then reduce heat and simmer 2 minutes. Purée prune and Armagnac mixture with sugar syrup in a food processor until very smooth, about 2 minutes. Add orange juice and remaining 1 1/4 cups cold water and pulse to combine. Force through a fine-mesh sieve into a bowl and discard solids. Chill until cold, about 1 hour. Freeze in ice cream maker, then transfer to an airtight container and put in freezer to harden, at least 2 hours. Cooks' note:Sorbet can be made 1 day ahead.