Absolute dimensions of eclipsing binaries .21. V906 Scorpii: a triple system member of M7

We present an analysis based on new uvby light curves and spectroscopic data of the detached triple-lined B-type type eclipsing binary V906 Sco. The uvby light curves are analysed with an extended version of the Wilson-Devinney program. The spectroscopic CCD observations are analysed with both the Sterne and the Lehmann-Filhes methods. We conclude from the combined analysis that the triple system V906 Sco is a member of the open cluster M 7 and that its B-type eclipsing components are still on the main sequence, at an age of (2.4 +/- 0.3) x 10(8) yrs, already close to the TAMS (especially the more massive one). The system is older than the time for circularization of the orbit, and the small eccentricity is probably caused by the third component, which, however, is unlikely to be the main responsible for the apsidal motion, probably more influenced by tidal and rotational deformation. We determine absolute dimensions of high precision (errors < 2%): M-A = 3.25 +/- 0.07, R-A = 3.52 +/- 0.04, M-B = 3.38 +/- 0.07 and R-B = 4.52 +/- 0.04, in solar units. The system is detached, with both components in synchronous rotation, and therefore representative for normal stars.