Hot stars born as rapid rotators are expected to induce meridional currents that mix hydrogen from the envelope into the core and return CNO-cycle processed material to the envelope, which should enhance the N at the surface at the expense of C and possibly also O depending on the ambient conditions. But the photospheric C and N abundances could also be influenced by mass transfer in a close binary system which spins up the mass gainer and deposits either processed or unprocessed material to its surface depending on just how much material has been peeled off the mass donor. We focus on the chemical composition of Be star photospheres to infer the present and past evolution of rapidly rotating early B stars. To mitigate the effects of gravity darkening and photospheric line blending on the abundances, we chose 8 Be stars with low vsin i that have good high-resolution FUV spectra in the IUE archive. We carried out a conventional NLTE abundance analysis of selected N iii, N i, and C iii lines in the FUV spectral region. We find clear evidence that the C iii 1176 Å multiplet is weak in the core region in most program stars, suggesting CNO processing. However, in all cases we infer a N abundance that is solar or less, raising a conundrum as to what happened to the “missing C.” Since a similar pattern of weak C yet normal N is also found in the mass gainer in some Algol binaries, there appears to be an emerging challenge to explain this apparent abundance anomaly. We speculate that the excess N from CNO processing might be converted into O (and perhaps on to Ne) by fusion with He in the hot but low-density regions either in the trail of ashes just outside the receding carbon-fusing core, or in He-shell flash regions, of a highly evolved mass loser in its final stage of mass transfer.

Abstract Optical interferometry is an observational technique that provides the highest spatial resolutions available in the optical. By interfering light from separate telescopes, and measuring the properties of the resulting interference pattern, it is possible to retrieve information about the night sky at spatial resolutions equal to the separation of the telescopes, overcoming the diffraction limit of a single telescope. In long baseline amplitude optical interferometry, the beams of light from the telescopes are transported to a central location and physically interfered. The interference is achieved via an instrument known as a beam combiner. In this review, I discuss the functionality of a beam combiner. I begin with a mathematical explanation of how interference fringes are produced and what information these interference fringes contain. This is followed by a discussion of how interference fringes are generated and measured in practise for the most common beam combination schemes, for both pupil plane and image plane combination and how these schemes can be realised in bulk optics or integrated optics. I also provide a detailed summary of the various design considerations that can affect the functionality of a beam combiner. Finally, I discuss current and future work in long baseline amplitude optical interferometry.