  The topology of the embedding of the coadjoint orbits of the unitary group
U(H) of an in-finite dimensional complex Hilbert space H, as canonically
determined subsets of the B-space T_s of symmetric trace class operators, is
investigated. The space T_s is identified with the B-space predual of the
Lie-algebra L(H)_s of the Lie group U(H). It is proved, that orbits con-sisting
of symmetric operators with finite rank are (regularly embedded) closed
submanifolds of T_s. An alternative method of proving this fact is given for
the `one-dimensional' orbit, i.e. for the projective Hilbert space P(H). Also a
technical assertion concerning existence of simply related decompositions into
one-dimensional projections of two unitary equivalent (orthogonal) projections
in their `generic mutual position' is formulated, proved, and illustrated.
