Heterotic String from Four-Dimensional Geometry
In any conformally invariant metric-connection theory of 4-dimensional spacetime, there are 26 ways that a body can distort. This follows from the absence of any preferred metric, and is equivalent to the ability of an observer to consider objects on any given world line to be distortion free. Within the simplest conformally invariant theory of distortions that allow metric compatible spaces as trivial solutions, we find the maximal classical solution. This solution corresponds to the bosonic sector of the heterotic string: 26 left-moving fields plus 10 right-moving fields.
Wheeler, James T., Heterotic String from four-dimensional geometry, University of North Carolina IFP, IFP-298-UNC (Sept, 1987).