  We present photometric and spectroscopic optical and near-infrared (NIR)
observations (Based on observations collected at the European Southern
Observatory, Chile (ESO ID 63.H-0493 and 64.H-0382)) taken during the outburst
of the recently flaring source V4641 Sgr = SAX J1819.3-2525 (in't Zand et al.,
2000), on September 1999. This source was independently detected as the RXTE
source XTE J1819-254 (Markwardt et al. 1999), and afterwards identified with
the variable object V4641 Sgr (Kato et al. 1999). It underwent a bright optical
outburst on 1999 Sept. 15.7 UT, going from magnitude 14 to 8.8 in the V-band
(Stubbings 1999), and reaching 12.2 Crab in the X-rays (Smith et al. 1999) and
Ks nearly 13. This outburst was therefore bright, but very brief, with an
e-fold decay time of 0.6 days. A radio source was resolved, making of V4641 Sgr
a new microquasar (Hjellming et al. 2000). We discuss the nature of this
system, showing that our observations suggest a distance farther than
previously derived from the radio observations (Hjellming et al. 2000). The
distance of the system would be between 3 and 8 kpc, the companion star being a
B3-A2 main sequence star. Another possibility is that the companion star is
crossing the Hertzsprung gap (type B3-A2 IV), and in this case the distance
cited above would be the minimum distance of the system. The system is
therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The
inconsistency regarding the distance between the radio and optical/NIR
observations could be explained by the detection of an interaction between
matter ejected before the X-ray outburst and the surrounding medium of the
source. If this is confirmed, this source could be added to the short list of
microquasars where such an interaction has been detected.
