4 Interstellar reddening

We have estimated the interstellar reddening to the source by using the correlation between the E(B-V) colour excess and the equivalent width of diffuse absorption features of interstellar origin (Herbig 1975). We used the lines $\lambda$4430, $\lambda$6376/79 and $\lambda$6613 only since a clean measurement of their equivalent width can be carried out. Due to the relative low resolution of our spectra some of the interstellar features appear blended with other interstellar or atmospheric lines. We find the mean colour excess to be $E(B{-}V)=0.9\pm0.2$, in agreement with an earlier measurement by M97 of $E(B{-}V)=0.8\pm0.2$ using the same spectroscopic method. However we find that this estimate suggests a significant $(V{-}K)\sim0.8$ colour excess when applied to the photometry from 1996 October (Table 3). By comparison we find that applying the photometrically derived reddening estimate from our data ( $E(B{-}V)=1.2\pm0.1$) results in no excess emission in the near IR. Additionally, the X-ray data support a higher value for the colour excess. From a BeppoSAX observation a hydrogen column density of ${\sim} (1.0\pm0.3) \times 10^{22}$ cm^-2 is found (Reig, private communication), implying $E(B{-}V)=1.9\pm0.6$ (Predehl & Schmitt 1995) - not statistically different from the value adopted from the photometric data. We therefore choose to adopt $E(B{-}V)=1.2\pm0.1$ for LS 5039 given that it is consistent with both the near-IR colours of, and hydrogen column density to, LS 5039.