  Dimension-five photon $(\gamma )$ scalar $(\phi)$ interaction terms usually
appear in the bosonic sector of unified theories of electromagnetism and
gravity. In these theories the three propagation eigenstates are different from
the three field eigenstates. The dispersion relation in an external magnetic
field shows that, for a non- zero energy $(\omega)$, out of the three
propagating eigenstates one has superluminal phase velocity $v_p$. During
propagation, another eigenstate undergoes amplification or attenuation, showing
signs of an unstable system. The remaining one maintains causality. In this
paper, using techniques from optics as well as gravity, we identify the energy
$(\omega)$ interval outside which $v_p \le c$ for the field eigenstates
$|\gamma_{\parallel} > $ and $ |\phi > $, and stability of the system is
restored. The behavior of group velocity $v_g$ is also explored in the same
context. We conclude by pointing out its possible astrophysical implications.
