  We have produced the colour-colour diagram of all the observations of 4U
1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996
to 2002) and found observations filling in a previously reported 'gap' between
the island and the banana X-ray states. We have made timing analysis of these
gap observations and found, in one observation, two simultaneous kHz
quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit
in the overall trend of the source. The 'lower' kHz QPO has a centroid
frequency of ~308 Hz. This is the lowest 'lower' kHz QPO frequency ever
observed in 4U 1728-34. The peak frequency separation between the 'upper' and
the 'lower' kHz QPO is 274+/-11 Hz, significantly smaller than the constant
value of ~350 Hz found when the 'lower' kHz QPO frequency is between ~500 and
800 Hz. This is the first indication in this source for a significant decrease
of kHz QPO peak separation towards low frequencies. We briefly compare the
result to theoretical models for kHz QPO production.
