Spectral classification of the hot components of a large sample of stars with composite spectra, and implication for the absolute magnitudes of the cool supergiant components.

A sample of 135 stars with composite spectra has been observed in the near-UV spectral region with the Aurelie spectrograph at the Observatoire de Haute-Provence. Using the spectral classifications of the coo l components previously determined with near infrared spectra, we obtained reliable spectral types of the hot components of the sample systems. The hot components were isolated by the subtraction method using MK standards as surrogates of the cool components. We also derived the visual magnitude differences between the components using Willstrop's normalized stellar flux ratios. We propose a photometric model for each of these systems on the basis of our spectroscopic data and the Hipparcos data. We bring to light a discrepancy for the G supergiant primaries between the visual absolute magnitudes deduced from Hipparcos parallaxes and those tabulated by Schmidt-Kaler for the GIb stars: we propose a scale of M-v-values for these stars in composite systems. By way of statistics, about 75% of the hot components are dwarf or subgiant stars, and 25% should be giants. The distribution in spectral types is as follows: 41% of B-type components, 57% of type A, and 2% of type F; 68% of the hot components have a spectral type in the range B7 to A2. The distribution of the DeltaM(v)-values shows a maximum near 0.75 mag.