Photometric Identification of Cool White Dwarfs

We investigate the use of a narrow-band DDO51 filter for photometricidentification of cool white dwarfs. We report photometric observations of 30known cool white dwarfs with temperatures ranging from 10,000 K down to verycool temperatures (<3500 K). Follow-up spectroscopic observations of a sampleof objects selected using this filter and our photometric observations showthat DDO51 filter photometry can help select cool white dwarf candidates forfollow-up multi--object spectroscopy by rejecting 65% of main sequence starswith the same broad--band colors as the cool white dwarfs. This technique isnot selective enough to efficiently feed single--object spectrographs. Wepresent the white dwarf cooling sequence using this filter. Our observationsshow that very cool white dwarfs form a sequence in the r-DDO vs. r-zcolor--color diagram and demonstrate that significant improvements are neededin white dwarf model atmospheres.Comment: Accepted for publication in A