We report the discovery of a new O–type double–lined spectroscopic binary with a short orbital period of 1.4 days .
We find the primary component of this binary , FO 15 , to have an approximate spectral type O5.5Vz , i.e .
a Zero–Age–Main–Sequence star .
The secondary appears to be of spectral type O9.5V .
We have performed a numerical model fit to the public ASAS photometry , which shows that FO 15 is also an eclipsing binary .
We find an orbital inclination of \sim 80 ^ { \circ } .
From a simultaneous light-curve and radial velocity solution we find the masses and radii of the two components to be 30 \pm 1 and 16 \pm 1 solar masses and 7.5 \pm 0.5 and 5.3 \pm 0.5 solar radii .
These radii , and hence also the luminosities , are smaller than those of normal O-type stars , but similar to recently born ZAMS O-type stars .
The absolute magnitudes derived from our analysis locate FO 15 at the same distance as \eta Carinæ .
From Chandra and XMM X-ray images we also find that there are two close X-ray sources , one coincident with FO 15 and another one without optical counterpart .
This latter seems to be a highly variable source , presumably due to a pre–main–sequence stellar neighbour of FO 15 .