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Published online by Cambridge University Press: 12 April 2016
Both SS Cyg and U Gem show enhanced soft X-ray emission (0.1–0.5 keV) during optical outbursts. In extended HEAO-1 pointings, it was discovered that a large fraction of this emission is pulsed (see Fig. 1). This confirms the general expectation that high energy pulsations are the ultimate source of the optical pulsations observed during outbursts of dwarf novae. For SS Cyg the average soft X-ray pulsation amplitude is 30%; this is to be contrasted with the upper limit of 10% to a hard X-ray (2–25 keV) pulsed component for the same observation (Swank 1979). Thus the origin of the oscillation is in soft X-rays, and the direct implication of such a large pulsed amplitude is that the oscillation itself must be an intrinsic part of the X-ray production mechanism.