KöMaL 2020 A781

We want to construct an isosceles triangle using a compass and a straightedge. We are given two of the following four data: the length of the base of the triangle ($a$), the length of the leg of the triangle ($b$), the radius of the inscribed circle ($r$) and the radius of the circumscribed circle ($R$). In which of the six possible cases will we definitely be able to construct the triangle?