New matrix formalism for heterotic string theory on a torus
  A new Lagrange formalism based on the use of a single scalar (d+1)X(d+1+n) matrix potential is developed for the low-energy heterotic string theory with n U(1) gauge fields compactified from d+3 to 3 dimensions on a torus. This formalism also includes three pairs on-shell defined scalar and vector matrix potentials of the dimensions (d+1)X(d+1), (d+1)X(d+1+n) and (d+1+n)X(d+1+n). All these potentials undergo linear transformations when the group of charging symmetries acts. 
Oleg V. Kechkin