A detailed photometric and spectroscopic analysis of cool ( T _ { eff } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 12 , 000 K ) white dwarf stars is presented .
The sample has been drawn from the Yale Parallax Catalog and from a proper-motion survey in the southern hemisphere .
Optical BVRI and infrared JHK photometry , as well as spectroscopy at H \alpha , have been secured for a sample of 152 white dwarfs .
The discovery of seven new DA white dwarfs , two new DQ white dwarfs , one new magnetic white dwarf , and three weak magnetic white dwarf candidates , is reported .
Our sample also identifies 19 known or suspected double degenerates .

The photometric energy distributions , the H \alpha line profiles , and the trigonometric parallax measurements are combined and compared against the predictions of model atmosphere calculations to determine the effective temperature and the radius of each object in the sample , and also to constrain the atmospheric composition .
New evolutionary sequences with carbon/oxygen cores with thin and thick hydrogen layers are used to derive stellar masses and ages .
The results are used to improve our understanding of the chemical evolution of cool white dwarfs .

We confirm the existence of a range in effective temperature between \sim 5000 and 6000 K where almost all white dwarfs have hydrogen-rich atmospheres .
Our sample shows little evidence for mixed H/He white dwarfs , with the exception of two helium-rich DA stars , and four ( possibly five ) C _ { 2 } H white dwarfs which have been interpreted as having mixed H/He/C atmospheres .
The observed sequence of DQ stars is found to terminate abruptly near 6500 K , below which they are believed to turn into C _ { 2 } H stars .
True DC stars slightly above this temperature are found to exhibit hydrogen-like energy distributions despite the lack of H \alpha absorption features .
The mean mass of our complete sample is 0.65 M _ { \odot } with a dispersion of \sigma \sim 0.20 M _ { \odot } .

Attempts to interpret the chemical evolution of cool white dwarfs show the problem to be complex .
Convective mixing is called upon to account for the increase of the non-DA to DA ratio below 12,000 K , as well as the reappearance of helium-rich stars below \sim 5000 K. The possible presence of helium in cool DA stars , the existence of the non-DA gap , and the nature of the peculiar DC stars are also explained in terms of convective mixing , although our understanding of how this mechanism works needs to be revised in order to account for these observations .
Given this chemical evolution uncertainty , it is not clear whether thick or thin hydrogen layer models should be used to determine cooling ages .
The oldest object in our sample is \sim 7.9 Gyr or \sim 9.7 Gyr old depending on whether thin or thick hydrogen layer models are used , respectively .