  The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass
X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer has stimulated
extensive studies of these sources. Recently, Osherovich & Titarchuk suggest a
new model for kHz QPOs and the related correlations between kHz QPOs 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 find that, if this model is
correct, 4U 1728-34 is possibly a strange star rather a neutron star.
