On the Nature of the Microquasar V4641 Sagittarii

We present photometric and spectroscopic optical and near-infrared (NIR)observations taken during the outburst on September 1999 of the source V4641 Sgr = SAX J1819.3-2525 (in't Zand, et al., 2000) = XTE J1819-254 (Markwardt et al., 1999a). We show that our observations suggest a distancebetween 4 and 8 kpc, the spectral type of the companion star beingconstrained between B3 and A2 V, making the system a High Mass X-rayBinary System (HMXB). In view of the radio and optical/NIR observations,it is possible that the ejecta of the source interacted with thesurrounding medium of the source.