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Elie Cartan has classified all compact symmetric spaces admitting a compact simple Lie group as their group of motion.There are 7 infinite series and 12 exceptional cases. The exceptional cases are related to real forms of exceptional Lie algebra. Most of these symmetric spaces admit at least one geometric interpretation usually in terms of complex and real Grassmannians and their generalizations to quaternions and octonions($\mathbb{H}$), octonions($\mathbb{\mathbb{O}}$), bioctonions ($\mathbb{C}\otimes \mathbb{O}$), quateroctonions ($\mathbb{H}\otimes \mathbb{O}$) and octooctonions ($\mathbb{O}\otimes \mathbb{O}$). See for example the Wikipedia's entry for Symmetric Spaces.

Two of the exceptional symmetric spaces, don't seem to have such a geometric interpretation as far as I know. In Cartan notation, these two spaces are called $EI$ and $EV$ and correspond respectively to the exceptional symmetric spaces $\frac{E_7}{SU(8) / \mathbb{Z}_2 }$ and $\frac{E_6}{USp(4)/ \mathbb{Z}_2}$ of respective rank and dimension$(4,42)$ and $(7,70)$.

Now that the stage is set, here is my question:

What is the geometric description of the symmetric spaces $\frac{E_{7}}{SU(8)/ \mathbb{Z}_2}$ and $\frac{E_6}{Sp(4)/ \mathbb{Z}_2}$?

References on the subject are also welcome. This question is motivated by an answer to this MO question.

In order to give a more precise idea of the kind of answer I expect, let me give some examples: the symmetric space $\frac{F_4}{\mathrm{Spin}(9)}$ is geometrically described as the Cayley projective plane $\mathbb{O}P^2$, the space $\frac{E_6}{\mathrm{SO}(10) \mathrm{SO}(2)}$ is geometrically the Caylay bioctonion plane $(\mathbb{C}\otimes \mathbb{O}) \mathbb{P}^2$ and the symmetric space $\frac{E_6}{F_4}$ is the space of isometrically equivalent collineations of the Cayley plane $\mathbb{O}\mathbb{P}^2$.

NB:These two spaces also show up as scalar manifolds in maximal supergravity theories, this is for example review in this article of Boya. But for this question, I won't consider supergravity as a geometric interpretation.

Updates

Richard Borcherds has provided an answer thanks to a reference to the book "Einstein manifolds", but the book gives the answer without any proofs or explanation. So we now have an answer but we don't understand it. So if anyone could help with explaining how "antichains" enter the story, it will be highly appreciated. I have put some extra information in the comments.

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Though I can't provide any direct answer, I wonder if you have looked at the book Spaces of Constant Curvature by J.A. Wolf or related papers? Contacting him directly at UC Berkeley might be useful, since he has gone beyond Elie Cartan in some respects and has worked for a long time with symmetric spaces. –  Jim Humphreys Aug 19 '10 at 15:03
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1 Answer

Section K of chapter 10 of Einstein manifolds by A. Besse gives geometric interpretations for all irreducible symmetric spaces. The 2 cases you ask about are apparently antichains in (H⊗Ca)P2, and (C⊗Ca)P2 (whatever this means).

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There's a wikipage on antichains, but how this relates to what's written in that table in Besse is unclear to me: en.wikipedia.org/wiki/Antichain –  José Figueroa-O'Farrill Aug 20 '10 at 8:50
    
Here $(C\otimes Ca)P^2$ and and $(H\otimes Ca)P^2$ are the projective planes constructed respectively over the algebra of bi-octonions $\mathbb{C}\otimes \mathbb{O}$ and quateroctinions $\mathbb{H}\otimes \mathbb{O}$. Here $Ca$ is just the algebra $\mathbb{O}$ of octonions. Now $E_6\simeq \mathrm{isom}((C\otimes Ca)P^2)$ and $E_7\simeq \mathrm{isom}((H\otimes Ca)P^2)$. I think in this context "antichain" is defined with using inclusion as the order, but I am not really sure so I still don't understand the answer. I have checked the book but it does not explain the result. –  JME Aug 22 '10 at 17:38
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