Hi there, Caroline! This article gives instructions for adding sourdough discard to a recipe, in which case you'd still want to add the yeast called for in a recipe as the discard will provide flavor but not much leavening power. You're welcome to experiment with using these tips for adding ripe sourdough starter to recipes in order to replace the yeast, but the rise times will also be a bit longer so you'll need to go more so by how the dough looks and feels rather than times that are given in a recipe. Happy experimenting! 

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