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We calculate properties of neutron stars such as mass and radius using a relativistic Dirac-Brueckner-Hartree-Fock approach. Modern meson-exchange potential models are used to evaluate the G matrix for asymmetric nuclear matter. For pure neutron matter we find the maximum mass to be M,x = 2.4MO for a radius R = 12 km, whereas with a proton fraction of 30% we find M a"= 2.1Mo for a radius R = 10.5 km, close to the experimental values. The implications are discussed.PACS numbers: 97.60.Jd, 21.65.+f

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A restricted repulsive barrier exists for equatorial photon motion in some Kerr-Newman-de Sitter blackhole spacetimes, due to an interplay between the rotation of the source and the cosmological repulsion. Quite surprisingly, photons with high positive and all negative values of their impact parameter can travel freely between the black-hole horizon and the cosmological horizon in these spacetimes. The special class of the Kerr-Newman-de Sitter spacetimes with restricted repulsive barriers is represented in the space of the parameters of the geometry. The area of the parameter space corresponding to this class is largest in the case of Kerr-de Sitter spacetimes. With the charge parameter of the spacetime growing, the area shrinks. In all of these cases, the mass parameter of the spacetime must be so high that the sizes of the black-hole and cosmological horizons are comparable. ͓S0556-2821͑98͒01918-3͔