\subsubsection{$SO(32)$ and $E_8\times E_8$} \subsubsection{Toroidal compactifications} \subsubsection{$\mathcal{N}=2$} [1]<ol><li>Barrett:1993yn</li><li>Marcus:1992wi</li><li>Berkovits:1993xq</li></ol> and Appendix of 7th chapter in [2]<ol><li>Cecotti:2023dnp</li></ol>.