Exact Four-Parameter Rotating NS--NS Vacuum in Double Field Theory
We construct an exact rotating vacuum of the NS--NS sector, equivalently a Double Field Theory vacuum. The construction applies compact $\mathbf{SO}(2)$ S-duality to a rotating Einstein--scalar seed. The solution has four independent parameters $\{\mathfrak{m},j,\mathfrak{q},\zeta\}$. To our knowledge, this is the first explicit rotating solution of the pure NS--NS vacuum equations with all three NS--NS fields $\{g,B,\phi\}$ determined analytically, independent dilaton and $H$-flux charges, and no Maxwell sector. The static limit is obtained by taking the rotation parameter $j\to 0$. In this limit the geometry is not the spherical Burgess--Myers--Quevedo solution. Instead, it is an axial Zipoy--Voorhees branch carrying $H$-flux, so an oblate deformation remains after rotation is switched off. This geometric memory is absent in pure general relativity and in Einstein--Maxwell--dilaton--axion. The two static branches nevertheless share the same $\ell=0$ parametrized post-Newtonian data $\{MG,\beta_{\mathrm{PPN}},\gamma_{\mathrm{PPN}},h\}$. They give two inequivalent NS--NS geometries at identical monopole charges, with the degeneracy lifted at $\ell=2$. At the Kerr horizon locus the outer shell is generically singular in curvature. Above the threshold $|\mathfrak{q}|>\sqrt{\mathfrak{m}^{2}-j^{2}}$, polar geodesics are repelled outward and the rotation axis becomes regular in curvature at the shell. On that axis the inverse metric $g^{\mu\nu}$ stays finite while the lower-index Riemannian metric components diverge, whereas off the axis the inverse metric itself diverges. This axis-local degeneracy may offer a setting for non-Riemannian geometry in Double Field Theory, where $g_{\mu\nu}$ is not fundamental and the $\mathbf{O}(D,D)$ variables $\{d,\mathcal{H}_{AB}\}$ remain well defined.