We examine AM CVn stars, ultra-short-period variables thought to have helium disks, from the standpoint of the disk instability model. By calculating the vertical structure of the helium accretion disks, we have confirmed that the helium accretion disks are thermally unstable, since their thermal-equilibrium curves exhibit the characteristic S-shaped form. We apply these results to AM CVn stars. We have found that the observed large-amplitude photometric variations in AM CVn stars can be

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... ed by the thermal-tidal instability model of helium accretion disks.