  We consider non-compact WZW models at critical level (equal to the dual
Coxeter number) as tensionless limits of gravitational backgrounds in string
theory. Special emphasis is placed on the Euclidean black hole coset
SL(2,R)_k/U(1) when k=2. In this limit gravity decouples in the form of a
Liouville field with infinite background charge and the world-sheet symmetry of
the model becomes a truncated version of W_\infty without Virasoro generator.
This is regarded as manifestation of Langlands duality for the SL(2,R)_k
current algebra that relates small with large values of the level in the two
extreme limits. However, the physical interpretation of the SL(2,R)_k/U(1)
coset model below the self-dual value k=3 remains elusive including the
non-conformal theory at k=2.
