The preceding invited paper by Sahade (1987) gave a balanced overall picture for the subject matter. I shall attempt to complement his talk in discussing two classes of atmospheric diagnostics of the evolutionary processes in interacting binary systems. These are: (I) Abnormal abundances of elements in the atmosphere resulting from the nuclear processes in the stellar interior. (II) Mass flow as a consequence of the evolution of either of the components and the Algol type binaries. (I)

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... Anomalies: There are several classes of objects with abundance anomalies, such as Am stars. At least some of these objects are known close binaries. In particular, most Am stars appear to be close binary stars. However, the physical causes of the metallic overabundances in those stars are still unclear. We shall in the following examine Algol type binaries for abundance anomalies. The best place to look for abundance anomalies is the atmosphere of the evolved late-type component. However, the star is typically fainter than the early type companion and is difficult to observe spectroscopically at high resolution for abundance analysis. An optimum time to observe its spectrum is during a primary (total) eclipse when the spectrum of only that star is visible. Regrettably, the total eclipse usually does not last long enough to obtain a good exposure at high resolution. Spectroscopic observations with large telescopes using imaging device may provide valuable data on the relative abundances of N, C and 0. A good candidate is S Cancri, whose late-type component has an estimated mass of only 0.18 solar mass while its unevolved hotter companion has an estimated mass of some 2.3 solar masses. The late-type star's atmosphere should reveal nuclear-processed material and overabundance of heavier elements. S Cnc may provide a rare opportunity to peel down the outlayer of an evolved star to see what it looks like inside. Using Digicon spectra obtained during the total phase of eclipse, Parthasarathy, Lambert and Tomkin (1983) concluded that the late-type components in U Cephei and U Sagittae had an underabundance of C and an overabundance of N, which they construed as the result of the conversion of C into N. 206 , available at https://www.cambridge.org/core/terms. https://doi.