  Minimal coupling leads to problems such as loss of causality if one wants to
describe charged particles of spin greater than one propagating in a constant
electromagnetic background. Regge trajectories in string theory contain such
states, so their study may allow us to investigate possible avenues to remedy
the pathologies. We present here two explicit forms, related by field
redefinitions, of the Lagrangian describing the bosonic states in the first
massive level of open superstrings in four dimensions. The first one reduces,
when the electromagnetic field is set to zero, to the Fierz-Pauli Lagrangian
for the spin-2 mode. The second one is a more compact form which simplifies the
derivation of a Fierz-Pauli system of equations of motion and constraints.
