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](http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=rm&paperid=2427&option_lang=eng) strata in $F$ are the equivalence classes with respect to any of the following three equivalence relations (which are shown to coincide).


1. $x\sim y$ iff the set of Plücker coordinates vanishing in $x$ is the same as that for $y$.
2. $x\sim y$ iff the set of $T$-fixed points contained in the orbit closure $\overline{Tx}$ is the same as that in $\overline{Ty}$.
3. $x\sim y$ 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?



Irreducibility of Gelfand-Serganova strata
The strata need not be irreducible.


Quoting page 2 of Knutson, Lam, Speyer (<https://arxiv.org/abs/0903.3694v1>): "the strata can have essentially any singularity [Mn88]. In particular, the nonempty ones need not be irreducible, or even equidimensional."


They are referencing ["Mnëv's universality theorem"](https://en.wikipedia.org/wiki/Mn%C3%ABv%27s_universality_theorem) here.


