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We present the results of a survey of optical and infrared photometry, as well as optical spectroscopy, of cool ($T_{\rm eff} \ \lower 0.5ex\hbox{$\buildrel < \over \sim\ $}7000$~K) white dwarf stars. Effective temperatures, surface gravities, and chemical abundances of helium and hydrogen are determined for individual objects by fitting the optical/infrared energy distributions with the predictions of new model atmospheres appropriate for these cool degenerate stars. We show that the atmospheric composition of the coolest stars can be inferred from the observed energy distributions, and that objects with mixed compositions ($N({\rm He})/N({\rm H})\sim 1$) are easily recognizable from their predicted strong infrared flux deficiency. Our study reveals that the dichotomy observed in hotter white dwarf atmospheres, namely that the photosphere is composed either of nearly pure hydrogen or of nearly pure helium, persists at low effective temperatures. In particular, no white dwarf star with a helium-to-hydrogen ratio near unity has been found, with the exception of the peculiar white dwarf LHS 1126. High signal-to-noise spectroscopy has revealed the presence of H$\alpha$ in many previously classified DC stars, and it has been detected in objects as cool as $T_{\rm eff} \sim4600$~K. The implications of our findings on physical mechanisms such as convective mixing and accretion from the interstellar medium will be discussed.
This work is supported in part by a Chr\'etien International Research Grant (PB), by the FONDECYT grant 92-880 (MTR), and by the NSF grant AST 93-15372 (SKL).
