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Saturn's rings exhibit a wealth of density and bending waves which are mostly attributable to resonances with external satellites. There is, however, a set of wavelike features in the C ring which do not fall near any known satellite resonances (Rosen etal. 1991; Baillie etal 2011). These waves appear to propagate radially inwards, suggesting that they arise at outer Lindblad resonances (OLRs), unlike the majority of satellite resonances. Marley & Porco (1993) proposed that the waves might be driven by resonances with Saturn's internal modes of oscillation, with predicted rotation periods of order 5 hours, but a lack of information on the angular frequency of the waves made specific identifications impossible. Using stellar occultation data from the Cassini VIMS instrument, we have now successfully measured both the azimuthal wavenumber and angular pattern speed for six of these waves. We find that all six waves are indeed consistent with density waves driven at OLRs with Saturnian f-modes. Our results should open a significant new window on the planet's interior structure and rotation state.