  An effective Hartree-Fock-Bogoliubov-type interaction is calculated for the
Hubbard-model in second order in the coupling by means of flow-equations. A
stability analysis is performed in order to obtain the transition into various
possible phases. We find, that the second order contribution weakens the
tendency for the antiferromagnetic transition. Apart from a possible
antiferromagnetic transition the d-wave Pomeranchuk instability recently
reported by Halboth and Metzner is usually the strongest instability. A newly
found instability is of p-wave character and yields a band-splitting. In the
BCS-channel one obtains the strongest contribution for $d_{x^2-y^2}$-waves.
Other types of instabilities of comparable strength are also reported.
