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We discuss an investigation of twenty low-resolution IUE spectra of the dwarf nova SS Cygni while the system was in outburst. The N V $\lambda$1240 and Si IV $\lambda$1400 resonance lines show velocity-shifted absorption while the C IV $\lambda$1550 resonance line displays a prominent P Cygni profile. Such lines are typical for nova-like variables and dwarf novae in outburst, and are an indication of strong, high-velocity outflows from these systems.
The C IV emission and absorption profiles both exhibit strong temporal variability. (Because of the difficulty in accurately determining the continuum, only the C IV line is considered in this study.) The emission line strength, as measured by the line equivalent width, displayed variations as large as a factor of $\sim$2 on time-scales as short as $\sim$ thirty minutes. Moreover, as the time resolution of the instrument is of this order it is very likely that the variability is of an even shorter time-scale. The modulation did not appear to be periodic. The absorption line-strength showed variations comparable in magnitude to the emission lines. However, this component appeared to be modulated on the system orbital period ($\sim$6.6 hours). Although the signal-to-noise ratio of these data is low, the coincidence of the apparent variation period with the system orbital period coupled with the fact that similar variations have been detected in at least three other dwarf novae, lead us to believe that the orbital periodicity is likely.
Our physical interpretation of the variability is that it results from an interaction of the high-velocity wind with a solar-type wind originating from the secondary in the system. An analogous interpretation has been given to explain similar behavior in the P Cygni profiles of some Wolf-Rayet binaries.
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