Two-hole bound states in modified t-J model

We consider modified $t-J$ model with minimum of single-hole dispersion at the points $(0,\pm \pi)$, $(\pm \pi,0)$. It is shown that two holes on antiferromagnetic background produce a bound state which properties strongly differs from the states known in the unmodified $t-J$ model. The bound state is d-wave, it has four nodes on the face of the magnetic Brillouin zone. However, in the coordinate representation it looks like as usual s-wave.