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We report microwave surface impedance measurements made on a single crystal
of PrOs_4Sb_12 across the frequency range 0.5 to 21 GHz. The penetration depth
data provide clear thermodynamic evidence for superconductivity arising in two
bands coupled by Josephson pair-tunneling, indicating two order parameters with
the same symmetry. Detailed conductivity spectra obtained using high-resolution
bolometry confirm this picture and extend it, revealing the itinerant nature of
the Pr f electrons through the observation of quasiparticles with heavily