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5 Conclusions

A Gaussian fit to the main peak in the histogram (Fig. 2) results in a formal maximum occurrence of 8% in our analysed (constant) stars with an amplitude exceeding 3.25 times the noise level at a frequency of 11.55 ${\rm d}^{\,-1}$. This frequency clearly is related to the rotation of HIPPARCOS. Considering the width of that fit one can question any published detections of variability, based on HIPPARCOS photometric data alone, with frequencies ranging from 9.78 to 13.32 ${\rm d}^{\,-1}$ and amplitudes exceeding the noise level only a few times.

The low frequency peaks in the analysis of presumably constant stars may result from at least three effects: not yet identified intrinsic variations of stars in our sample, instrumental drifts and aliasing. In the present investigation we tried to simulate a "blind'' analysis of HIPPARCOS photometry where it is impossible to disentangle such effects on the basis of a statistical treatment. Large periods obviously have to be checked independently from HIPPARCOS data.


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{ms1709f4.eps} \end{figure} Figure 4: HR-diagram of our candidate stars, using different symbols for stars with and without spurious periodicities ($\pi $ in mas).

The optimum frequency interval for detecting low frequency variability with HIPPARCOS, claimed by van Leeuwen (1997) to range from 0.5 to 11 ${\rm d}^{\,-1}$, is questioned by our histogram (Fig. 2) at least for amplitudes comparable to the noise level. Such an interval seems to be restricted rather to 2 to 9 ${\rm d}^{\,-1}$. However, it is comfort to see that the orbit frequency of 2.25 ${\rm d}^{\,-1}$ has only marginal effects on determining intrinsic stellar photometric periods.

Acknowledgements
We acknowledge helpful comments and suggestions from L. Eyer and L. M. Freyhammer. Financial support was received from the Bundesministerium für Bildung, Wissenschaft und Kultur (project EXTRACTOR), the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (project P14984), and the ASTROVIRTEL Project which is run by the ESO/ST-ECF Archive and funded by the European Commission under contract 5FP HPRI-CT-1999-00081.

Note added in proof: E. Paunzen brought our attention to a paper by Koen (2001) where the author lists in his Table 3 stars from the "unsolved'' part of the HIPPARCOS catalogue which are multiperiodic according to his criterium. It is interesting to note that allstars in his Table, except of 5 (from 36), were selected on the grounds of frequencies coinciding with what we call "spurious'' frequencies for our sample of presumably constant stars. We take this evidence as an independent indication for critical frequency ranges in the HIPPARCOS photometry, where HIPPARCOS data alone are insignificant and have to be supplemented by independent observations.


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