In this letter , we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet ( UV ) as the second component commonly identified in the far ultraviolet ( FUV ) as the ” accretion belt ” .

We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario .
We consider detailed multiwavelength observations and in particular the recent FUSE observations of VW Hyi which confirm the presence of a second component ( the ” accretion belt ” ) in the FUV spectrum of VW Hyi in quiescence .
The temperature ( \approx 50 , 000 K ) and rotational velocity ( > 3 , 000 km s ^ { -1 } ) of this second FUV component are entirely consistent with the optically thick region ( \tau \approx 1 ) located just at the outer edge of optically thin boundary layer in the simulations of Popham ( 1999 ) .

This second component contributes 20 % of the FUV flux , therefore implying a boundary layer luminosity : L _ { BL } = 2 \times ( 0.2 \times L _ { UV } + L _ { X - ray } ) = 0.6 \times L _ { disc } , while the theory ( Kluźniak 1987 ) predicts , for the rotation rate of VW Hyi ’ s WD , L _ { BL } \approx 0.77 L _ { disc } .
The remaining accretion energy ( < 0.1 L _ { acc } ) is apparently advected into the star as expected for optically thin advection dominated boundary layers .
This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by ( Pandel , Córdova & Howell 2003 ) , from which we deduced here that the alpha viscosity parameter in the boundary layer region must be as small as \alpha \approx 0.004 .