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1. Introduction

Stars with spectral types later than tex2html_wrap_inline1096F5 show magnetic surface activity, chromospheres and coronae like the Sun. Stars with earlier spectral types appear to have a weaker outer atmosphere, supposedly due to the shallowness or absence of outer convection zones in these stars (Schrijver 1993, and references therein). As part of a study of the onset of stellar activity along the main sequence, we have used ROSAT all-sky survey data to determine X-ray fluxes of a sample of 173 main-sequence stars, with spectral types between A8 and G2. In order to derive detailed quantitative information on the location of these stars in the Hertzsprung-Russell diagram, we have also made Walraven five-colour photometric observations.

The sample studied here was selected from the Bright Star Catalogue (BSC; Hoffleit & Jaschek 1982, lists almost all stars with tex2html_wrap_inline1098 mag) according to the following criteria:

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Spectral type between A8 and G2; no spectral peculiarities noted; not double in spectral type classification (e.g., HR 32 with spectral type F2V+F6V is excluded).
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Luminosity class V;
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Right ascension between tex2html_wrap_inline1100 and tex2html_wrap_inline1102, or between tex2html_wrap_inline1104 and tex2html_wrap_inline1106, declination south of tex2html_wrap_inline1108 (defining the region on the sky visible during the appointed observation times);
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Binaries for which both components occurred in the BSC are excluded, if the separation is less than tex2html_wrap_inline1110.

The selected sample is listed in Table 1, excluding five stars for which no (photometric and X-ray) data are available (HR 591, HR 5542, HR 6593, HR 8245, HR 8735). Four stars have not been observed in the Walraven photometric system, and for 11 stars we have no ROSAT data available. These 15 stars with incomplete data are indicated as such in Table 1.

This paper reports the results of the ROSAT and Walraven observations. Spectroscopic observations and determinations of the rotational velocities for the sample stars have been described in Groot et al. (1996).


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