  We study $AdS_3 \times S^1 \times Y$ supersymmetric string theory backgrounds
with Neveu-Schwarz-Neveu-Schwarz flux that are dual to ${\cal N}=2$
superconformal theories on the boundary. We classify all worldsheet vertex
operators that correspond to space-time chiral primaries. We compute space-time
chiral ring structure constants for operators in the zero spectral flow sector
using the operator product expansion in the worldsheet theory. We find that the
structure constants take a universal form that depends only on the topological
data of the ${\cal N}=2$ superconformal theory on $Y$.
