  We report on a joint BeppoSAX/RossiXTE observation of the Z-type low mass
X-ray binary Cyg X-2. The source was in the so-called high overall intensity
state and in less than 24 hours went through all three branches of the Z-track.
The continuum x-ray spectrum could be described by the absorbed sum of a soft
thermal component, modeled as either a blackbody or a multicolor disk
blackbody, and a Comptonized component. The timing power spectrum showed
several components including quasiperiodic oscillations in the range 28-50 Hz
while the source was on the horizontal branch (horizontal branch oscillation;
HBO). We found that the HBO frequency was well correlated with the parameters
of the soft thermal component in the x-ray spectrum. We discuss implications of
this correlation for models of the HBO.
