Simple sl_(n+1)-module structures on U(h)

We study the category M consisting of U(sl_{n+1})-modules whose restriction to U(h) is free of rank 1, in particular we classify isomorphism classes of objects in M and determine their submodule structure. This leads to new sl_{n+1}-modules. For n=1 we also find the central characters and derive an explicit formula for taking tensor product with a simple finite dimensional module.