On the groups of c-projective transformations of complete K\'ahler manifolds

We show that for any complete connected K\"ahler manifold the index of the group of complex affine transformations in the group of c-projective transformations is at most two unless the K\"ahler manifold is isometric to complex projective space equipped with a positive constant multiple of the Fubini-Study metric. This establishes a stronger version of the recently proved Yano-Obata conjecture for complete K\"ahler manifolds.
Comment: 24 pages; a few more minor corrections; to appear in Annals of Global Analysis and Geometry