An astrometric calibration of the MV-Porb relationship for cataclysmic variables based on Hubble Space Telescope fine guidance sensor parallaxes

We present new, high-precision astrometric parallaxes for three cataclysmic variables: WZ Saggitae, YZ Cancri, and RU Pegasi, obtained using the Fine Guidance Sensors on the Hubble Space Telescope. The addition of these three parallaxes to the existing data set allows us to examine the outburst luminosities for dwarf novae spanning the orbital period range 1.36 hr less than or equal to P-orb less than or equal to 8.99 hr. We find that, after correcting for the orbital inclination, there is a simple linear relationship between the absolute visual magnitude at outburst and the orbital period. Such a relationship suggests that the only difference in the outbursts between long- and short-period systems is the actual physical size of their accretion disks. When we compare the rare outbursts of three intermediate polar systems (EX Hydrae, TV Columbae, and V1223 Sagittarii), and the visual high states of the nova-like variable RW Tri, four other cataclysmic variables with published HST parallaxes, with the new M-V-P-orb relationship derived for the dwarf novae, we find that the absolute visual magnitudes during the "outbursts'' of these four systems attain the luminosity predicted for their orbital period. This suggests that these short-lived outbursts may also be steady state accretion events like the eruptions of dwarf novae.