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Published online by Cambridge University Press: 14 August 2017
A substantial part of our understanding of the physical nature of stars rests upon observations of visual binaries. In order to secure a complete interpretation of a binary system we require these data: semi-major axis, parallax, period, magnitudes, colours, spectral types and velocities. It goes without saying that for very few systems are all these desiderata available, largely through technical difficulties of observing two images in close proximity to one another - and these difficulties are particularly serious in carrying out photometry. Nevertheless, the usefulness of photometry of binaries is so great that no effort should be spared in resolving the observational and technical problems.