  We consider a generalization of a duality symmetric model proposed by Schwarz
and Sen. It is based on enlarging the model with a dynamical vector field being
a time-like component of a local Lorentz frame. This allows one to preserve the
manifest Lorentz invariance of the model in flat space--time. The presence of
this field is regarded as a relic of gravitational interaction which respects
the general coordinate invariance in curved space--time but breaks the local
Lorentz symmetry in tangent space down to its spatial subgroup.
