| Summary: | This work describes several self-contained pieces of research which are all linked by their connection with optical interferometry. First is an instrumentational project involving the selection, installation and subsequent testing of second-generation optical detectors for the Cambridge Optical Aperture Synthesis Telescope (COAST). These new detectors extended the magnitude limit of COAST by over a magnitude in the I band, increasing its range of observable astronomical targets and making it the most sensitive separate-element interferometer in this band. Also described were two observational projects. The first was carried out using COAST and was made possible by the improved sensitivity of the instrument after installation of the new detectors. It involved observations of two Be stars, Gamma Cassiopeiae and Zeta Tauri, in a narrow Hα band. Non-zero closure phases were measured in both cases, indicating deviations from axisymmetry in the circumstellar discs of the two stars. The first true map of Gamma Cassiopeiae was presented. The second observational project involved measurements of the proto-planetary nebula IRC+10216 made with the UK InfraRed Telescope (UKIRT), using the interferometric technique of speckle masking to achieve close to diffraction-limited imaging. The results include the first ever K-L map and K-band polarisation map of this source. From these and other observations, we found evidence that the morphology of the complex inner dust cloud differs from the models suggested by most other authors, and the star is completely obscured in both the K and L bands.
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