The spectroscopic and astrometric parallaxes of three dwarf novae: The nature of the secondary stars of U Geminorum, SS Aurigae, and SS Cygni

We compare the parallaxes of three dwarf novae obtained using the Fine Guidance Sensors on the Hubble Space Telescope with their ground-based infrared spectroscopic parallaxes to examine the nature of their secondary stars. We have used new optical spectroscopy of MK spectral-type standards to rework the spectroscopic parallaxes of the reference frames used by Harrison et al. for each dwarf nova. The recalibrated parallaxes for U Gem, SS Aur, and SS Cyg have then been corrected for the Lutz-Kelker bias. We present the proper motions of the three objects, finding space motions consistent with an old disk population. New moderate-resolution infrared spectroscopy of the three dwarf novae, covering the spectral range 2.18 to 2.30 mum, have been used to determine the spectral types of the secondary stars. We find an M1 spectral type for the secondary star of SS Aur, an M4 for U Gem, and a K4 for SS Cyg, in agreement with previous determinations. Using these spectral types and the astrometric parallaxes, we have deconvolved the spectral energy distributions for all three dwarf novae. Within the errors, the JHK photometry of SS Aur is consistent with a normal M1 V supplying its entire infrared luminosity. However, the infrared luminosities of both U Gem and SS Cyg are too high to be supplied by normal main-sequence stars of their observed spectral types. We investigate models for the nonstellar components of all three dwarf novae by subtracting various cool star and white dwarf spectra from the observed data. For U Gem and SS Aur we find that power-law residual spectra can be extracted for the nonstellar components of these two systems. When this analysis is performed for SS Cyg, however, we find that two additional luminous components remain.