  Low-mass X-ray binaries containing weakly magnetized neutron stars may be
divided into two classes, Z and atoll sources, based in large part upon the
patterns which they trace in plots of two X-ray colors. In this paper, we
examine color-color diagrams of nine atoll sources and five Z sources using
data from the Rossi X-ray Timing Explorer. The five-year span of data we have
examined is significantly longer than those of previous studies. We find that
the previous clear distinction between atoll and Z sources may be an artifact
of incomplete sampling, as those atoll sources which are sampled over a wide
dynamic range in intensity (greater than a factor of 80) trace three-branched
color-color patterns similar to the tracks for which Z sources are named. This
suggests that the two classes are more intimately related than previously
realized, which also has been suggested based upon recent timing studies.
However, detailed differences in the spectral behavior of the two classes argue
against any simple unification.
