Automorphs of indefinite binary quadratic forms and K3-surfaces with Picard number 2

Every even indefinite binary form occurs as the Picard lattice of some K3-surface. For these there is an explicit description for the group of isometries. This group is always infinite, while the automorphism group of the corresponding K3-surface can be finite. To single out the automorphs induced by actual automorphisms of the K3-surface requires some extensions of the classical results and, when carried out, leads to an explicit description of all possible automorphism groups of ``general'' K3's with Picard number two.