6 Conclusion

In course of our high resolution spectroscopic survey of Herbig Ae/Be (HAeBe) binary stars, we observed 42 objects with $m_V\,<\,11$: 6 stars exhibit Li I 6708Å absorption line attributed to a cooler companion (4 are new detections), and for 7 other stars, radial velocity variations are recorded (4 are new detections). TY CrA is a particular triple system for which both binarity criteria (i.e. radial velocity variation and presence of Li I absorption line) are observed. Four stars unlikely appear to belong to the HAeBe class.

The Li I line, not observed in our HAeBe candidates from Table 5, is not only an indicator of binarity, but may also help in some cases to classify a star as either a T Tauri or as a HAeBe star. Depending on the presence or not of the Li I 6708Å line, the distinction, between the class of young low mass stars and the class of young intermediate mass stars, could be inferred thanks to this criterion. This distinction has been proposed here for some peculiar cases, such as RY Ori or T Cha.

Within our reduced sample, the observed binary frequency for short-period spectroscopic HAeBe systems ($fb \approx 10$%) is roughly comparable to the one of T Tauri or MS stars. The true binary frequency is probably higher considering to the present biases working against the detection of spectroscopic binary stars.

Due the limitation in magnitude, this systematic search would be greatly completed with the use of 8 m-class telescope, to have access of fainter HAeBe stars and enlarge our sample, and to interferometric technics under development. It is emphasized that the spectroscopic binaries discussed in the paper deserves further observations in order to obtain more information about the secondary component (luminosity, age) and possibly to retrieve the masses: their careful study by various means (spectroscopic follow-up, careful photometric monitoring, lunar occultation, interferometric measurements...) are encouraged.

Acknowledgements

This research has made use of the Simbad database, operated at CDS, Strasbourg, France. We warmly thanks the INSU and ESO observing programs committees for generous telescope time allocations and the OHP and ESO telescope teams for their kind support. P.C. thanks P. Prugniel for early discussion at OHP and D. Queloz for his efficient and friendly presence during the first nights with ÉLODIE. We would like to express our gratitude towards S. Allain, D. Mouillet, J.C. Augereau and L. Siess who kindly performed some of the observations, and also to C. Ounnas and J. Rodriguez for some service observations. We thank J. Bouvier for providing useful comments, and H. Beust who specially modified his program for this study. Finally, we are grateful to the referee, Dr. Andrea Richichi, for useful comments on this work and careful reading of the manuscript.