5 Conclusion

In this paper, periodic structures in variable star light curves from the Hipparcos mission were studied with robust variogram estimators. Several problems were investigated, such as bin positions, bin widths and robustness properties. Some specific applications to the Hipparcos mission were discussed, such as spurious periods detections and measurement noise. The method can be successful for detecting time-scales of semi-regular stars of the Hipparcos mission. Although bin positions are specific to the Hipparcos mission, the whole method can be easily applied to other irregular time series.

Acknowledgements

We would like to thank S. Paltani, M. Grenon and F. Mignard for their interesting discussions and comments. We also thank the referee X. Luri and D. Barthès for their remarks. The first author is grateful to the Swiss Society of Astronomy and Astrophysics for its financial support. The second author is grateful to the Swiss Federal Institute of Technology of Lausanne where the main part of this work was done.