Scoop out the insides of the oranges and strain the juice, pressing the pulp firmly against the sieve. Put the juice in a pan with the sugar and heat gently to dissolve. Leave to cool. Scoop the flesh out of the passion fruit and strain, again pressing the pulp against the sieve.
Mix the passion fruit juice into the orange mixture. Freeze in an ice cream machine. (Alternatively transfer to an airtight container and freeze for 2 hours. Remove from the freezer, using a form or electric whisk, puree until the sorbet is smooth. Return to the freezer and leave until the sorbet hardens. This should take 4 hours).
