Abstract
Starting from type IIB string theory on an ADE singularity, the (2, 0) little string arises when one takes the string coupling gs to 0. In this setup, we give a unified description of the codimension-two defects of the little string, labeled by a simple Lie algebra $$ \mathfrak{g} $$
g
. Geometrically, these are D5 branes wrapping 2-cycles of the singularity, subject to a certain folding operation when the algebra is non simply-laced. Equivalently, the defects are specified by a certain set of weights of $$ {}^L\mathfrak{g} $$
L
g
, the Langlands dual of $$ \mathfrak{g} $$
g
. As a first application, we show that the instanton partition function of the $$ \mathfrak{g} $$
g
-type quiver gauge theory on the defect is equal to a 3-point conformal block of the $$ \mathfrak{g} $$
g
-type deformed Toda theory in the Coulomb gas formalism. As a second application, we argue that in the (2, 0) CFT limit, the Coulomb branch of the defects flows to a nilpotent orbit of $$ \mathfrak{g} $$
g
.
Conformal block expansion in celestial CFT