Symmetric subgroup orbit closures on flag varieties: their equivariant geometry, combinatorics, and connections with degeneracy loci

We give explicit formulas for torus-equivariant fundamental classes of closed K-orbits on the flag variety G/B when G is one of the classical groups SL(n,C), SO(n,C), or Sp(2n,C), and K is a symmetric subgroup of G. We describe parametrizations of each orbit set and the combinatorics of its weak order, allowing us to compute formulas for the equivariant classes of all remaining orbit closures using divided difference operators. In each of the cases in type A, we realize K-orbit closures as universal degeneracy loci of a certain type, involving a vector bundle V over a scheme X equipped with a flag of subbundles and a further structure determined by K. We describe how our equivariant formulas can be interpreted as formulas for such loci in the Chern classes of the various bundles on X.