First simultaneous optical and extreme-ultraviolet observations of the quasi-coherent oscillations of SS Cygni

Using extreme-ultraviolet (EUV) photometry obtained with the Extreme Ultrav iolet Explorer (EUVE) satellite and UBVR optical photometry obtained with t he 2.7 m telescope at McDonald Observatory, we have detected quasi-coherent oscillations (so-called dwarf nova oscillations) in the EUV and optical fl ux of the dwarf nova SS Cyg during its 1996 October outburst. There are two new results from these observations. First, we have for the first time obs erved "frequency doubling": during the rising branch of the outburst, the p eriod of the EUV oscillation was observed to jump from 6.59 to 2.91 s. Seco nd, we have for the first time observed quasi-coherent oscillations simulta neously in the optical and EUV. We find that the period and phase of the os cillations are the same in the two wave bands, finally confirming the long- held assumption that the periods of the optical and EUV/soft X-ray oscillat ions of dwarf novae are equal. The UBV oscillations can be simply the Rayle igh-Jeans tail of the EUV oscillations if the boundary layer temperature kT (bb) less than or similar to 15 eV and hence the luminosity L-bb greater th an or similar to 1.2 x 10(34)(d/75 pc)(2) ergs s(-1) (comparable to that of the accretion disk). Otherwise, the lack of a phase delay between the EUV and optical oscillations requires that the optical reprocessing site lies w ithin the inner third of the accretion disk. This is strikingly different f rom other cataclysmic variables, where much or all of the disk contributes to the optical oscillations.