  We report the discovery of a new pyrochlore oxide superconductor RbOs2O6. The
compound crystallizes in the same beta-pyrochlore structure as the recently
discovered superconductor KOs2O6, where Os atoms form a corner-sharing
tetrahedral network called the pyrochlore lattice with Rb or K atoms in the
cage. Resistivity, magnetic susceptibility and specific heat measurements on
polycrystalline samples evidence a bulk superconductivity with Tc = 6.3 K.
