On the nature of the compact star in 4U 1728-34

The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-ma ss X-ray binaries (LMXBs) with the Rossi X-Ray Timing Explorer has stimulat ed extensive studies of these sources. Recently, Osherovich & Titarchuk sug gested a new model for kHz QPOs and the related correlations between kHz QP Os and low-frequency features in LMXBs. Here we use their results to study the mass-radius relation for the atoll source 4U 1728-34. We fmd that, if t his model is correct, 4U 1728-34 is possibly a strange star rather than a n eutron star.