... A. Jorissen

Senior Research Associate, FNRS, Belgium.

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... D. Pourbaix

Research Associate, FNRS, Belgium.

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... deviation

The analysis of the single-star fits for the whole Hipparcos Catalogue reveals that the F2 statistics has a mean 0.21 and standard deviation 1.08 (ESA 1997). This indicates that the formal errors have been slightly underestimated. Since the same formal errors are used to compute  $\chi^2_{\rm T}$, the F2 statistics for the Thiele-Innes fits has been assumed to have the same parameters as for the single-star fits. Consequently, the 5% threshold corresponds to F2 = 2.37.

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