The southern, bright and massive late B-type (B9V + B9V, 
,
, eccentric orbit) detached eclipsing binary V906 Sco (see
Table 1 (click here)) was discovered as eclipsing by Koelbloed (1959), in a
photometric study of M7. The system is also a triple-lined
spectroscopic binary (Lacy & Evans 1979; Alencar et al. 1997).
Moderate proximity effects are present in the light curves, which show
well defined and unequal minima, the primary minimum having a depth of
, 
deeper than the secondary one in the colour y.
Figure 1: y magnitude differences and b-y and u-b colour index
differences V906 Sco-HD 
obtained at ESO, with the
theoretical light curves (Alencar et al. 1997)
The eclipsing components have different masses, and are well inside their Roche-lobes, with the secondary (the star closest to the observer during the primary minimum) being more massive, larger, but cooler than the primary. V906 Sco is one of the rare systems with components still on the main sequence, presenting different masses and with the more massive component very close to the terminal age main sequence (TAMS, Alencar et al. 1997). This, together with the fact that V906 Sco is a member of the open cluster NGC 6475, makes the system very important for the control of modern models of stellar evolution.
Here we present the first accurate and complete light
curves of V906 Sco. Medium- (18Å/mm) and high- (6Å/mm)
dispersion CCD coudé spectra have also been secured. These data and
a study of V906 Sco based on them (published separately, Alencar et al.
1997) yield precise absolute dimensions (
, 
; 
, 
) and
confirm that the system is a member of Messier 7 (NGC 6475), with an
age of 
years. The eclipsing components
constitute a visual pair with the third companion, and the slightly
eccentric eclipsing orbit probably presents apsidal motion (Alencar
et al. 1997).
