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 ()