To keep the notations simple I'll restrict my attention to the complete flag variety although the question should be equally valid for partial flag varieties. Consider $G=SL_n(\mathbb C)$ with Borel $B\subset G$, Cartan $T\subset B$ and flag variety $F=G/B$. The Plücker embedding $$F\subset\mathbb P(\wedge^1\mathbb C^n)\times\ldots\times\mathbb P(\wedge^{n-1}\mathbb C^n)$$ equips $F$ with Plücker coordinates $\{X_{i_1,\ldots,i_k}\}$.

The Gelfand-Serganova strata in $F$ are the equivalence classes with respect to any of the following three equivalence relations (which are shown to coincide).

- $x\sim y$ iff the set of Plücker coordinates vanishing in $x$ is the same as that for $y$.
- $x\sim y$ iff the set of $T$-fixed points contained in the orbit closure $\overline{Tx}$ is the same as that in $\overline{Ty}$.
- $x\sim y$ iff for any Borel $B'\supset T$ the orbits $B'x$ and $B'y$ coincide, i.e. $x$ and $y$ lie in the same Schubert cell with respect to $B'$. In other words, the strata are the nonempty intersections of $n!$ Schubert cells, one for every Borel containing $T$.

Unfortunately, as shown in the above paper, the closure of a stratum is not necessarily a union of strata. This rules out many nice properties one could hope for in such a setting. However, I still have a feeling that the strata (and their closures) might always be irreducible. Is that so and has this been discussed in the literature?