Abstract
We determine in this work the fundamental properties of three eclipsing massive O-type binaries, namely FO 15, CPD-59 2603 and CPD-59 2635, which are members of the young open cluster Trumpler 16 in the Carina Nebula. FO 15 was recently discovered by Niemela et al. (2006) to be an O5.5 Vz + O9.5 V short period binary system. CPD-59 2603 is a hierarchical triple system (Rauw et al. 2001, hereafter R01), made up of an O-type eclipsing binary (O7 V+O9.5 V) and a B0.2 IV type star. CPD-59 2635 was found by Albacete Colombo et al. (2001, hereafter AC01) to be an O-type spectroscopic binary system (O8 V + O9.5 V) with `Struve-Sahade' effect. These binaries show the signature of the Vz luminosity class of very young stars recently arrived to the Zero Age Main Sequence (ZAMS), namely HeIIλ4686 absorption stronger than other HeII lines.
