  We have performed simultaneous X-ray and optical spectroscopic observations
of the Low Mass X-ray Binary Cygnus X-2. We have used a new data system
attached to the Low Resolution Imaging Spectrograph instrument on Keck II to
obtain spectra with a mean time resolution of 72.075 milliseconds, simultaneous
with pointed X-ray observations using the PCA onboard XTE. In this paper, we
have analysed the variability in both wavebands on timescales of 16 seconds.
During our observations Cygnus X-2 covered all three branches of the Z-curve,
allowing us to study how the changes in X-ray spectral state affect the optical
emission. As the optical flux rises the X-ray intensity first rises on the
Horizontal Branch (0<S_z<1) but then falls on the Normal Branch (1<S_z<2) and
Flaring Branch (2<S_z<3), where S_z is a rank number characterising the
position on the Z-curve. This linear increase in the optical flux with S_z
indicates the optical flux is a good predictor of the accretion rate (possibly
normalized by its own long-term average) inferred from the Z-state S_z. We have
used this correlation to decompose the total X-ray count-rate into three
distinct spectral components.
