1 Introduction

The early type star LS 5039 has been proposed as the optical counterpart to the X-ray source RX J1826-1450 by Motch et al. (1997, hereafter M97), who classify it as O7V((f)) on the basis of low resolution blue end spectroscopy. Subsequent observations by Martí et al. (1998, hereafter Ma98) suggested that LS 5039/RX J1826-1450 was also associated with a persistent, moderately variable unresolved radio source with a non thermal radio spectrum of spectral index $\alpha \sim-0.47$ (adopting the convention $S_{\nu} \propto \nu^{\alpha}$).

Ribó et al. (1999, henceforth R99) present a detailed timing and spectral analysis of X-ray data obtained with the Rossi X-ray Timing Explorer (RXTE). They find the timing analysis shows no evidence for either pulsed emission, or periodic variability between 2-200 d, which might arise from orbital modulation of the X-ray flux. Furthermore, they analysed four months of daily observations of the 8.3 and 2.25 GHz flux of LS 5039, obtained as part of a long term monitoring campaign with the Green Bank Interferometer (GBI), but found no evidence for periodic variability between 2-50 d, or well defined flaring activity in these data.

More recently Paredes et al. (2000) succesfully resolved the radio emission as a bipolar jet, confirming LS5039 as one of the few radio emitting High Mass X-ray Binaries (hereafter HMXB) known in the galaxy. Given their rather heterogeneous nature, further observations of RX J1826-1450/LS 5039 are clearly of interest. Here we present new optical (4100-7400 Å) and near-IR (1.5-2.2 $\mu$m) spectroscopic data obtained between 1995-2000 enabling a more accurate spectral classification for the system. Photometric observations from 1997-2000 are also presented. In combination with published data and our spectroscopic observations these allow us to search for medium-long term variability. Finally we also undertake a more detailed analysis of the complete 8.3 and 2.25 GHz GBI datasets than has previously been published.