  We have discovered kiloHertz quasi-periodic oscillations (kHz QPOs) in five Z
sources: GX 340+0, Cygnus X-2, GX 17+2, GX 5-1, and Scorpius X-1. In all
sources the properties of these kHz QPOs are very similar and closely related
to the position of the sources on the Z track traced out in the X-ray
color-color diagram and the hardness-intensity diagram. The frequencies of the
kHz QPOs increase when the sources move from the left end of the horizontal
branch to horizontal/normal branch vertex, thus with inferred mass accretion
rate. Only for Scorpius X-1 the kHz QPOs have been observed down the normal
branch unto the flaring branch. The strength and the FWHM of the
higher-frequency kHz QPOs decrease with mass accretion rate, but when the
lower-frequency kHz QPOs are detected the strength and the FWHM of this QPO
stay approximately constant with mass accretion rate. In Scorpius X-1 the
frequency separation between the kHz QPOs decreases with mass accretion rate,
but in the other Z sources the separation remains approximately constant,
although a similar decrease in peak separation as found in Scorpius X-1 can not
be excluded.
