Clean and peel the strawberries and rhubarb. Cut the fruit into pieces of roughly the same size and let them steep together in a saucepan for a few hours. Then bring the fruits to the boil, stirring constantly, simmer for about 20 minutes. Skim off the rising foam. Add the preserving sugar and white rum to the fruit puree, briefly bring to the boil again. Fill the still hot jam into sterile preserving jars and seal them airtight.