  We compute the fundamental group of an open Richardson variety in the
manifold of complete flags that corresponds to a partial flag manifold. Rietsch
showed that these log Calabi-Yau varieties underlie a Landau-Ginzburg mirror
for the Langlands dual partial flag manifold, and our computation verifies a
prediction of Hori for this mirror. It is log Calabi-Yau as it isomorphic to
the complement of the Knutson-Lam-Speyer anti-canonical divisor for the partial
flag manifold. We also determine explicit defining equations for this divisor.
