Observations of quasi-coherent soft X-ray oscillations in U Geminorumand SS Cygni.

Using the HEAO 1 low-energy experiment, the authors have twice detected pulsations in the very soft X-ray flux of SS Cygni and once in U Gem. The pulsations were not strictly coherent; coherence time scales were deduced for the pulsations. During the first X-ray pointing at SS Cygni the observed 9 s oscillation had a coherence time of ≡30 pulse cycles. During the U Gem and second SS Cyg pointings, the decay times of the ≡27 s and ≡11 s X-ray oscillations were only one to two pulse cycles, which is of the same order as the decay times of longer period "quasi-coherent" optical pulsations detected in these stars. It is possible that the X-ray pulsations result from changes in the rate of the accretion flow.