  We analyze Bosonic, Heterotic, and Type II string theories compactified on a
generic torus having constant moduli. By computing the hamiltonian giving the
interaction between massive string excitations and $U(1)$ gauge fields arising
from the graviton and Kalb-Ramond field upon compactification, we derive a
general formula for such couplings that turns out to be universal in all these
theories. We also confirm our result by explicitly evaluating the relevant
string three-point amplitudes. From this expression, we determine the
gyromagnetic ratio $g$ of massive string states coupled to both gauge-fields.
For a generic mixed symmetry state, there is one gyromagnetic coupling
associated with each row of the corresponding Young Tableau diagram. For all
the states having zero Kaluza Klein or Winding charges, the value of $g$ turns
out to be $1$. We also explicitly consider totally symmetric and mixed symmetry
states (having two rows in the Young diagram) associated with the first
Regge-trajectory and obtain their corresponding $g$ value.
