The southern, reasonably bright and massive early type (O7V + O9V, 
, 
, circular orbit) detached eclipsing binary
V 3903 Sgr (see Table 1 (click here)) was discovered as eclipsing by
Cunha et al. (1990), after reports of variations in the
brightness of the system by Cousins (1973) and Clariá
(1976). The system is also a double lined spectroscopic binary
(Conti & Alschuler 1971; Niemela&Morisson
1988; Vaz et al. 1993, 1997). Therefore,
V 3903 Sgr was included
in our program for attainment of high precision differential photometry,
as a good candidate for absolute dimension determinations. Strong
proximity effects are present in the light curves, which show shallow
but well defined and unequal minima, the primary minimum having a depth
of 
, 
deeper than the secondary one in the y band.
V 3903Sgr is one of the rare massive systems
with components still on the main sequence.
Amongst these, only EMCar (Andersen&Clausen 1989) and
YCyg (Simon et al. 1994; Hill & Holmgren 1995)
have masses 
determined, together with the radii, to the
accuracy desirable for tests of evolutionary models:
. However, both EMCar and YCyg
have nearly equal mass components, what makes difficult the
control of theoretical evolutionary tracks. It turned out that
V 3903Sgr is the system with the most massive primary, with the largest
mass difference between the components, and the one closest
to the theoretical ZAMS (Vaz et al. 1997) amongst those with
the highest reliable absolute dimensions known to date.
In this paper we present the first accurate and complete light curves
of V 3903 Sgr. Medium- (18 Å/mm) and high- (6 Å/mm) dispersion
CCD coudé spectra have also been obtained. A study
based on these data, including times of minimum and a period
analysis, (published separately, Vaz et al. 1997) yields
precise absolute dimensions (
, 
; 
,
, solar units) and concludes that the system is
a member of the Lagoon Nebula complex, very close to the young open
cluster NGC 6530, and with an age between 1.7 and 
years.
Table 1: Catalogue data and standard 
indices for
V 3903 Sgr and the comparison stars. The indices for V 3903 Sgr
are for phases 
(uvby) and 
(
)