The luminosity of the boundary layer is reduced below the standard value if the white dwarf is rotating (3). While in U Gem, the luminosities of BL and disk agree , Mauche finds for SS Cyg a disconcertingly low value of LBL/Ldisk = 0.05±0'03 . This result suggests that the white dwarf rotates with a very short period of ^9 s, which agrees with the period of its quasi-coherent oscillations. The origin of the soft X-ray oscillations seen in quiescence and in outburst is not finally settled (for a review that concentrates on the X-ray emission see ).
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