2003MNRAS.345L..35M

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 Δν 274±11 Hz, significantly smaller than the constant value of Δν∼ 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 compare the result briefly to theoretical models for kHz QPO production.