  In this paper, we describe an example of a hyperkaehler quotient of a Banach
manifold by a Banach Lie group. Although the initial manifold is not
diffeomorphic to a Hilbert manifold (not even to a manifold modelled on a
reflexive Banach space), the quotient space obtained is a Hilbert manifold,
which can furthermore be identified either with the cotangent space of a
connected component of the restricted Grassmannian or with a natural
complexification of this connected component, thus proving that these two
manifolds are isomorphic hyperkaehler manifolds. In addition, Kaehler
potentials are computed using Kostant-Souriau's theory of prequantization.
