Lower limit on radius as a function of mass for neutron stars

A model-independent limit on neutron star radius as a function of mass based only on well-accepted principles is derived. We discuss our limit in connection with a recent interpretation of x-ray pulsations from SAX J1808.4-3658 as indicating a strange-star candidate, and show that this object can also be a normal neutron star, though one whose central core has very high density. The most plausible high density phase of hadronic matter, which is also expected to be very compressible, is quark matter. So an alternative to the strange star interpretation of SAX J1808.4-3658 is that it is a hybrid neutron star.