We present accurate optical and radio astrometry for the unresolved contact binary system Cyg OB2 No. 5 and its weaker component to the northeast. While the radio and optical positions of the primary component (taken to be the unresolved contact binary) agree within observational error (∼70 mas), we find that the weaker radio component does not agree in position with the second optical component of the system, but falls in between the two optical components. We conclude that the weaker radio component of this system is not associated with the secondary star but appears to be synchrotron emission produced at the shock interaction zone between the winds of the stars. Differential B-and V-band CCD and Hipparcos photometry was used to derive the approximate spectral type (B0 V-B2 V) of the secondary. Ram pressure arguments in the colliding-wind model give a massloss rate for the secondary star consistent with the expected one, considering the earliest spectral type.

Abstract. An investigation is carried out on the Wilson-Bappu effect in the Mg ii k line at 2796.34Å. The work is based on a selection of 230 stars observed by both the IUE and HIPPARCOS satellites, covering a wide range of spectral types (F to M) and absolute visual magnitudes (−5.4 ≤ MV ≤ 9.0). A semi-automatic procedure is used to measure the line widths, which applies also in the presence of strong central absorption reversal. The WilsonBappu relationship here provided is considered to represent an improvement over previous recent results for the considerably larger data sample used, as well as for a proper consideration of the measurement errors. No evidence has been found for a possible dependence of the WB effect on stellar metallicity and effective temperature.

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Among the astrophysical researches which will largely benefit from observation from the Antarctic Plateau, we consider those which require high-quality, continuous, daylight, uninterrupted observation in the optical–near-IR region. The systematic study of the stellar micro-oscillations and surface activity of individual objects, as well as of stellar associations extended for periods of weeks, will make a fundamental contribution to our knowledge of stellar structure and will ultimately provide the physical parameters of the stars. A similar consideration applies for the short-period (a few hours to a few days) binaries, which include the W UMa contact binaries. These objects are subject to continuously varying light curves associated with surface activity and secular effects, which cannot be adequately studied from the currently available ground telescopes. Finally, the exceptional seeing and low background will allow the systematic search for and study of supernovae in clusters of galaxies at cosmological distances, which will improve our knowledge of the geometry of the Universe at z ~ 0·7–1.

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