  We want to describe the triplets (\Omega, (g), \mu) where (g) is the
(co)metric associated to some symmetric second order differential operator L
defined on the domain \Omega of R^d and such that L is expandable on a basis of
orthogonal polynomials of L_2(\mu), and \mu is some admissible measure. Up to
affine transformation, we find 11 compact domains in dimension 2, and also give
some non--compact cases in this dimension.
