Optical spectroscopy of X-Mega targets in the Carina nebula - VI. FO15: a new O-type double-lined eclipsing binary

We report the discovery of a new O-type double-lined spectroscopic binary with a short orbital period of 1.4 d. We find the primary component of this binary, FO15, to have an approximate spectral type O5.5Vz, i.e. a zero-age main-sequence (ZAMS) star. The secondary appears to be of spectral type O9.5V. We have performed a numerical model fit to the public All Sky Automated Survey photometry, which shows that FO15 is also an eclipsing binary. We find an orbital inclination of ~80°. From a simultaneous light curve and radial velocity solution, we find the masses and radii of the two components to be 30 +/- 1 and 16 +/- 1M_solar and 7.5 +/- 0.5 and 5.3 +/- 0.5R_solar. 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 FO15 at the same distance as η Carinæ. From Chandra and XMM X-ray images, we also find that there are two close X-ray sources: one coincident with FO15 and the other one without optical counterpart. The latter seems to be a highly variable source, presumably due to a pre-main-sequence stellar neighbour of FO15.