4 Conclusion

The discovery that eight more classical Cepheids have spectroscopic companion, also supports the facts (Szabados 1995) that the incidence of binaries among classical Cepheids is larger than 50 per cent, and there are lots of spectroscopic binaries among Cepheids fainter than 8th magnitude whose duplicity remains to be discovered.

The modern radial velocity spectrometers (CORAVEL, ELODIE, etc.) attached to a telescope of about 1m diameter are capable of measuring stars of 12-13th magnitude within a reasonably short time, and the precision of the radial velocity determination allows the detection of very small orbital effects (less than one km s^-1 change in the $\gamma$-velocity), if homogeneous datasets covering several seasons are available. The shortest known orbital period for classical Cepheids is slightly longer than one year (Szabados 1992), while the other extremum, the longest still detectable orbital period is as long as several decades (T Mon).

The earlier radial velocity data of limited precision are useful for revealing the orbital motion if there is only a short dataset of recent radial velocity values, which is the case for the stars discussed in the present paper. The next step, then, is the extensive study of these newly recognized spectroscopic binaries. For the time being, none of the orbital elements can be determined from the available data. There are, however, several pieces of information that can be deduced from the pattern of the $\gamma$-velocity change, such as the short orbital period of TX Mon and the several-year-long orbital period is adequate to the observations of VW Pup. All Cepheids whose binary nature is reported here show considerable amplitude of $\gamma$-velocity variation (only V495 Mon may be an exception). For the sake of information, the new radial velocity data on the eight new SB-Cepheids obtained by the CORAVEL and ELODIE spectrographs are listed in Table 9.

The detection of numerous spectroscopic binaries among Cepheids justifies that the time-consuming project of collecting radial velocity data of Cepheids is an observational programme worthy to be carried out because binary Cepheids are key-objects for both astrophysics (stellar evolution) and cosmology (distance scale).

  

Table 9: Radial velocity data for the programme stars (for V495 Mon: ELODIE-data, for the other seven Cepheids: CORAVEL-data)

Acknowledgements

This project has been partly financed by Hungarian OTKA-project T022946. L.Sz. is grateful to Dr. Mária Kun for her remarks contributing to the improvement of the manuscript.