  We study the fourth-order stability of the triangular libration points in the
absence of resonance for the three-body problem when the infinitesimal mass is
affected not only by gravitation but also by light pressure from both
primaries. A comprehensive summary of previous results is given, with some
inaccuracies being corrected. The Lie triangle method is used to obtain the
fourth-order Birkhoff normal form of the Hamiltonian, and the corresponding
complex transformation to pre-normal form is given explicitly. We obtain an
explicit expression for the determinant required by the Arnold-Moser theorem,
and show that it is a rational function of the parameters, whose numerator is a
fifth-order polynomial in the mass parameter. Particular cases where this
polynomial reduces to a quartic are described. Our results reduce correctly to
the purely gravitational case in the appropriate limits, and extend numerical
work by previous authors.
