  Using a rotationally invariant version of the slave-boson approach in spin
space we analyze the stability of stripe phases with large unit cells in the
two-dimensional Hubbard model. This approach allows one to treat strong
electron correlations in the stripe phases relevant in the low doping regime,
and gives results representative of the thermodynamic limit. Thereby we resolve
the longstanding controversy concerning the role played by the kinetic energy
in stripe phases. While the transverse hopping across the domain walls yields
the largest kinetic energy gain in the case of the insulating stripes with one
hole per site, the holes propagating along the domain walls stabilize the
metallic vertical stripes with one hole per two sites, as observed in the
cuprates. We also show that a finite next-nearest neighbor hopping $t'$ can tip
the energy balance between the filled diagonal and half-filled vertical
stripes, which might explain a change in the spatial orientation of stripes
observed in the high $T_c$ cuprates at the doping $x\simeq 1/16$.
