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T. E. Harrison (NMSU), P. Szkody (U. Washington), J. J. Johnson (NMSU)
We have recently completed an HST Fine Guidance Sensor program that measured the first high-precision (± 0.5 mas) parallaxes for three dwarf novae. The true distances for all three objects turned out to be substantially larger than predicted by indirect methods. As part of this program, we obtained infrared spectra of the three program dwarf novae, and several other cataclysmic variables (e.g., WZ Sge), to examine the spectral types of their secondary stars. These new infrared spectra have revealed that the secondary stars of cataclysmic variables (CVs) are peculiar. Several of the secondary stars, such as those for U Gem and SS Cyg, have CO features (at \lambda = 2.29 \mum) that are much weaker than they should be, given their spectral types. In other CVs, atomic absorption lines due to sodium, calcium or magnesium are weaker than expected. We attribute the weakness of the CO feature in U Gem to a deficit of carbon in the photosphere of its secondary star. A photospheric carbon deficit in the secondary stars of CVs has been predicted from models of normal, close binary star evolution.
This research has been partially supported by funding from the AAS Small Research Grant Program.