Tensionless strings on $AdS_3 \times S^3 \times S^3 \times S^1$

Abstract: We propose that string theory on ${\rm AdS}_3 \times {\rm S}^3 \times {\rm S}^3 \times {\rm S}^1$ with $\textit{two}$ units of NS-NS flux through each of the two $3$-spheres is exactly dual to the symmetric orbifold of $2$ bosons and $8$ free fermions. Here, one of the two bosons describes the compact ${\rm S}^1$, while the other one corresponds to the radial (non-compact) direction of ${\rm AdS}_3$. Unlike the analogous situation for ${\rm AdS}_3 \times {\rm S}^3 \times \mathbb{T}^4$, the description makes sense both in the NS-R as well as the hybrid formulation, and we explain the duality in both frameworks.