Neutron Star Mass Determinations
  I review attempts made to determine the properties of neutron stars. The focus is on the maximum mass that a neutron star can have, or, conversely, the minimum mass required for the formation of a black hole. There appears to be only one neutron star for which there is strong evidence that its mass is above the canonical 1.4 solar masses, viz., Vela X-1, for which a mass close to 1.9 solar masses is found. Prospects for progress appear brightest for studies of systems in which the neutron star should have accreted substantial amounts of matter. 
M. H. van Kerkwijk (Utrecht University)