| Summary: | This thesis describes searches for very low mass stars (0.3 > m/m > 0.08) and brown dwarfs (0.08 > m/m > 0.01) in the Pleiades and Carinae galactic open clusters using COSMOS automatic measures of Schmidt photographic plates. A broad introduction defines the problem and relates research in this area to other branches of astronomy; the second chapter presents a detailed review of the current observational and theoretical status of the subject; and the third chapter briefly describes the observational techniques employed. The results are presented in chapters four and five. The Pleiades results indicate the presence of significant numbers of very low mass stars and possible brown dwarfs with the distinct possibility of discovering more in the near future; two techniques for extracting Pleiades member stars from the field (star counts and proper motions) are described and the cluster luminosity function is found to be strikingly similar in shape to that found for the field stars near the sun. A list of probable Pleiades members, the faintest of which may be brown dwarfs, is presented in the appendix. The derived mass function, although uncertain, indicates that brown dwarfs are present in the cluster. The preliminary analysis of the field around the Carinae cluster highlights the problems of crowded field astronomy using these techniques. Although this cluster does not appear to be as rich as the Pleiades, the results obtained so far indicate possible very low mass main sequence membership that could be investigated further with more plate material. Finally, these and other results are discussed in chapter six and suggestions for further work are presented. Appendices at the end compile data on the least luminous objects known, present photoelectric calibration sequences for the plate material and reproduce reprints of two papers derived from this work.
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