Optical spectroscopy of X-Mega targets in the Carina nebula -: VI.: FO 15:: 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 FO 15 is also an eclipsing binary. We find an orbital inclination of similar to 80 degrees 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 circle dot and 7.5 +/- 0.5 and 5.3 +/- 0.5R circle dot. 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 n Carinae. From Chandra and XMM X-ray images, we also find that there are two close X-ray sources: one coincident with FO 15 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 FO 15.