Denominators of Eisenstein cohomology classes for GL2 over imaginary quadratic fields

We study the arithmetic of Eisenstein cohomology classes for symmetric spaces associated to GL₂ over imaginary quadratic fields. We prove in many cases a lower bound on their denominator in terms of an L-value of a Hecke character providing evidence for a conjecture of Harder that the denominator is given by this L-value. Furthermore, we exibit conditions under which the restriction of the classes to the boundary is integral.