1995A&A...303L...1L

Though the existence of strange stars have been conjectured, it is very difficult to distinguish them from neutron stars, since both strange stars and neutron stars share similarities in many observable phenomena. But the mass-radius (M-R) relations of strange stars are different from those of neutron stars. The M-R relation of an X-ray pulsar can be determined semi-empirically from the observations of its spin variation and cyclotron spectral line. Here we calculate the semi-empirical M-R relation of the X-ray pulsar Her X-1. Comparing the result with the theoretical interior models of a neutron star and of a strange star respectively, we find that a strange star model is more consistent with Her X-1 and suggest it is a strange star.