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Imaginary reflections and discrete symmetries in the heterotic Monster

Let $\mathbb{M}$ be the Monster finite simple group. We give an interpretation of certain discrete symmetries of a family of heterotic string compactifications to $1 + 1$ dimensions in terms of discrete symmetries of the Monster Lie algebra $\frak m$, and more generally Carnahan's family of Monstrous Lie algebras $\frak m_g$, for $g\in\mathbb{M}$ of Fricke type. We relate a Weyl group-type reflection $w_{\im}$, with respect to an imaginary simple root, to a composition of T-duality, time reversal, and parity reversal transformations in the compactified heterotic string. The transformation $w_{\im}$ also has a natural permutation action on the BPS states of the compactified heterotic string.