  We study numerically the effective pair potential between star polymers with
equal arm lengths and equal number $f$ of arms. The simulations were done for
the soft core Domb-Joyce model on the simple cubic lattice, to minimize
corrections to scaling and to allow for an unlimited number of arms. For the
sampling, we used the pruned-enriched Rosenbluth method (PERM). We find that
the potential is much less soft than claimed in previous papers, in particular
for $f\gg 1$. While we verify the logarithmic divergence of $V(r)$, with $r$
being the distance between the two cores, predicted by Witten and Pincus, we
find for $f>20$ that the Mayer function is hardly distinguishable from that for
a Gaussian potential.
