  We consider the $GSO(-)$ sector of the open superstring using the formalism
with four-dimensional hybrid variables. This sector is defined by the world
sheet hybrid variables $(\theta^{\alpha},\bar{\theta}^{\dot\alpha})$ with
antiperiodic boundary conditions. The corresponding spectrum of states and
conditions for physical vertices are described. In particular we construct
explicitly the lower level $GSO(-)$ vertex operators corresponding to the
tachyon and the massless fermions. Using these new vertices, the tachyon and
massless sector contribution to the superstring field theory action of
Berkovits is evaluated. In this way we have included the Ramond sector and we
end by discussing some features of the action
