The Sydney University Stellar Interferometer, SUSI, a modern version of a Michelson stellar interferometer, will be capable of measuring the angular diameters of stars down to V ≈ 8.0, for all spectral types. The resolution of the 640 m baseline — <0.1 milliarcsec at optical wavelengths — includes reasonable sample sizes of all spectral types and luminosity classes to this magnitude limit.

The angular diameter data require complementary photometric, spectrophotometric and also spectroscopic data for the determination of fundamental parameters of single and multiple stars. This paper discusses the accuracy required of these data for some of the main problems to be tackled by SUSI. A selection of the programmes planned for this unique instrument is listed below, where the need for complementary data is also indicated.

1. • The measurement of the changes of angular diameters of Cepheids and other pulsating stars such as Mira variables. Comparison with the linear changes determined from radial velocity curves will enable their distances and luminosities to be determined by an essentially geometric method.

2. • The measurement of the angular sizes of the orbits and of one or both components of spectroscopic binary stars. This effectively makes them ‘visual’ binaries so that the orbital inclination, i, may be determined. When these data are combined with the velocity curve solutions which include (mass)sin3i and (semi – majoraxis)sin i, we may determine masses and linear radii of one or both components and also the distances to the systems. Also, the light curve of an eclipsing binary provides information on the ellipticities and radii of the components and on the eccentricity of the orbit — information which may be used to aid the analysis of the SUSI data. In addition, the determination of the light curve (some bright systems have not been observed for up to one or two decades) is the quickest method of finding the current phase of its orbit.

3. • The establishment of the total-flux-based temperature scale of all spectral types and luminosity classes from O to M. This requires accurate photometry and spectrophotometry over as wide a wavelength range as possible; it therefore also requires observations from space observatories, as was also necessary for the analysis using the Narrabri intensity interferometer data (Code et al., 1976).

4. • The measurement of diameters of stars with shells or extended atmospheres, such as Be and Wolf Rayet stars, at different wavelengths — in particular, in the emission lines.