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1. Introduction

The tex2html_wrap_inline1882Bootis stars are a class of metal-poor Populationi A-type stars. Although the prototype was already described by Morgan et al. (1943), the definition as a separate class among chemically peculiar (CP) stars is still controversial. tex2html_wrap_inline1884Bootis stars occupy the same parameter space in a Hertzsprung-Russell-diagram as do "normal" A-type stars, CP1 and cool CP2 stars. Figures1 (click here), 4 (click here) and 5 (click here) illustrate this coincidence for Geneva and Strömgren colour indices. The plotted colour indices were taken from Hauck & Mermilliod (1990), Handler (1995) for the Strömgren photometric system and from the Geneva database for the Geneva photometric system. The values for the tex2html_wrap_inline1886Bootis stars are listed in Tables 2 and 3.

  figure227
Figure 1: B2-V1 versus d. The solid lines are the standard relations after Golay (1980), crosses are "normal stars" from Gray & Garrison (1987, 1989a,b), open squares are CP-stars brighter than seventh magnitude from Renson et al. (1991), filled triangles are tex2html_wrap_inline1892Bootis stars

The evolutionary status of tex2html_wrap_inline1894Bootis stars is not clear yet. The two theories discussed in the literature exclude each other and observations are not yet conclusive enough for a decision. These theories involve either accretion of interstellar matter, or a combination of diffusion and mass loss. tex2html_wrap_inline1896Bootis stars would therefore provide excellent tests for processes which play a considerable rôle in modern astrophysics.

  figure234
Figure 2: B2-V1 versus tex2html_wrap_inline1900. The solid line is the standard relation after Golay (1980). Symbols are the same as in Fig. 1

In an attempt to compare properties of various stars which were claimed to be tex2html_wrap_inline1902Bootis\ stars and to determine unquestionable classification criteria, it became obvious that confusion exists frequently whether a given star belongs to the group or not. When we started this project, there was not even a sample of commonly accepted tex2html_wrap_inline1904Bootis stars available which was large enough for a sound statistical analysis of group properties. We felt it therefore necessary to define first a sample of unambiguously identified tex2html_wrap_inline1906Bootis stars, based on a consistent and independent classification, before any further steps can be taken to increase the number of group members and to define their common properties, as well as to develop a consistent tex2html_wrap_inline1908Bootis evolutionary theory.


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