Put egg yolks with the sugar in a bowl and beat until thick. Put the light cream in a heavy−based saucepan, and heat to just below simmering point then beat into the egg yolks. Return to the rinsed pan and cook over low heat, stirring constantly, until thickened. Set aside to cool, stirring occasionally. Remove the flesh from the avocados and purée with the orange zest and juice and put in a bowl. Beat in the cooled custard and fold in the whipped cream. Spoon into a container, cover and freeze until just becoming firm. Beat well in a bowl. Whip the egg whites until stiff but not dry and fold into the avocado mixture. Spoon the mixture back into the container. Cover and freeze until firm. About 30 minutes before serving, transfer the ice cream to the refrigerator. When serving, top each bowl of ice cream with a lemon slice.