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   81351
   SS Cygni is one of the most well-observed dwarf novae, displaying
   optical outburst every 30-60 days. In SS Cyg, the masses of both the
   white dwarf and the secondary star are measured precisely, and the
   relation between the mass accretion rate and the visual magnitude (mV)
   is calibrated with the accuracy of 20%.  By comparing mV and the X-ray
   luminosity, we attempt to measure the critical mass accretion rate
   which discriminate the optically thin and thick accretion disc. The
   critical mass accretion rate for a binary whose system parameters are
   well known like SS Cyg will pose a significant constraint on the theory
   of the accretion disc such as ADAF or so.
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