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Eight-dimensional non-geometric heterotic strings with gauge algebra$\mathfrak{e}_8\mathfrak{e}_7$ were constructed by Malmendier and Morrison asheterotic duals of F-theory on K3 surfaces with $\Lambda^{1,1}\oplus E_8\oplusE_7$ lattice polarization. Clingher, Malmendier and Shaska extended theseconstructions to eight-dimensional non-geometric heterotic strings with gaugealgebra $\mathfrak{e}_7\mathfrak{e}_7$ as heterotic duals of F-theory on$\Lambda^{1,1}\oplus E_7\oplus E_7$ lattice polarized K3 surfaces. In thisstudy, we analyze the points in the moduli of non-geometric heterotic stringswith gauge algebra $\mathfrak{e}_7\mathfrak{e}_7$, at which the non-Abeliangauge groups on the F-theory side are maximally enhanced. The gauge groups onthe heterotic side do not allow for the perturbative interpretation at thesepoints. We show that these theories can be described as deformations of thestable degenerations, as a result of coincident 7-branes on the F-theory side.From the heterotic viewpoint, this effect corresponds to the insertion of5-branes. These effects can be used to understand nonperturbative aspects ofnongeometric heterotic strings. Additionally, we build a family of ellipticCalabi-Yau 3-folds by fibering elliptic K3 surfaces, which belong to theF-theory side of the moduli of non-geometric heterotic strings with gaugealgebra $\mathfrak{e}_7\mathfrak{e}_7$, over $\mathbb{P}^1$. We find thathighly enhanced gauge symmetries arise on F-theory on the built ellipticCalabi-Yau 3-folds.

Unbroken $E_7\times E_7$ nongeometric heterotic strings, stable degenerations and enhanced gauge groups in F-theory duals