7 Conclusions

We have presented optical and near-IR spectroscopy that confirm that the mass donor in LS 5039 is an early O6.5V((f)) star. Long term observations show that there are no long term trends in the optical properties of the O6.5V((f)) star. Likewise, while there are day to day variations of a few hundredths of a magnitude in the optical lightcurve we find no coherent periodicities or long term trends that might be the result of orbital modulation; the most likely explanations for the low level optical variability are changes in the mass loss rate or non radial pulsations in the system primary. Variations of ${\sim}0.4$ mag are apparent in the H and K band photometry between 1995-2000; at present there are no obvious mechanisms for such variability within the system. Variability is also present in both X-ray and radio wavebands. However there is no evidence for significant flaring or periodic modulation; we note that if orbital modulation of the radio lightcurve was present above ${\sim}4$ per cent of the mean flux level at 2.25 GHz (as is seen in Cyg X-1) it would have been detectable. Although we cannot confirm the orbital separation suggested by Paredes et al. the lack of orbital modulation in the radio, X-ray and optical lightcurves is consistent with a rather large orbital separation and hence long orbital period. We therefore attribute the differences in the properties of the LS 5039 and the similar system Cyg X-1 to a combination of the weaker stellar wind and larger orbital separation of LS 5039.

Acknowledgements

This paper is partially based on observations collected at the European Southern Observatory, Chile. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology-Hellas and the Max-Planck-Institut für Extraterrestrische Physik. P. Reig acknowledges support via the European Union Training and Mobility of Researchers Network Grant ERBFMRX/CT98/0195.