On the radio emitting high mass X-ray binary LS 5039

We present new optical - near-IR spectroscopic and photometric observations of the newly discovered galactic microquasar LS 5039, which indicate a cla ssification for the mass donor in the system of O6.5V((f)). Optical spectro scopy and photometry shows no variability over a timescale of years, and we find no evidence of modulation by, or emission from the compact companion in these data. However significant photometric variability (similar to0.4 m ag) is present in the H and K bands between 1995-2000. Such variability has been observed in other radio bright X-ray binaries where it has been attri buted to synchrotron emission from the jet. However, given the non-thermal spectral index of the radio emission in LS 5039 this explanation appears un likely, predicting a near-IR flux similar to3 orders of magnitude too small to contribute significantly at such wavelengths. Nightly optical photometr y over a 21 day period between 2000 May-June reveals variability at a level of a few hundredths of a magnitude, with no periodicity or long term trend visible. Likewise, while the radio lightcurves show moderate variability ( similar to 20 per cent of the mean flux density) we find no evidence of per iodic modulation - Monte Carlo simulations constrain any such periodic vari ability to <4 per cent modulation at 2.25 GHz. The differences in behaviour between LS 5039 and Cygnus X-1 - the most closely related radio emitting H igh Mass X-ray Binary - are likely to be a result of the weaker stellar win d and probable greater orbital separation of LS 5039 compared to Cyg X-1.