  Due to the lack of both parity and time reversal symmetries, the Rashba
superconductors CePt_3Si, CeRhSi_3, and CeIrSi_3, in the presence of a magnetic
field, are unstable to helical (single plane-wave) order. We develop a
microscopic theory for such superconductors and examine the stability of this
helical phase. We show that the helical phase typically occupies most of the
magnetic field-temperature phase diagram. However, we also find that this phase
is sometimes unstable to a multiple-q phase, in which both the magnitude and
the phase of the order parameter are spatially varying. We find the position of
this helical to multiple-q phase transition. We further examine the density of
states and identify features unique to the helical phase.
