Physical elements of the eclipsing binary

For years, δ Orionis was considered a normal binary with an O9.5 II primary exhibiting apsidal-line advance. However, radial-velocity curves of both binary components have been derived from the IUE and optical spectra using the cross-correlation technique, and surprisingly low masses of 11.2 and 5.6 $M_{\odot}$ were found. We obtained new spectra in the red spectral region and new UBV photometry. Using all published photometry and radial velocities, we deduced more accurate orbital and apsidal line periods. The main result of this paper is to show that the observed line spectra of δ Orionis are composed of the lines of the O9.5 II primary and a similarly hot tertiary, while the lines of a cooler B-type secondary are too faint to be detected in the available spectra. The character of the light curve (low-amplitude partial eclipses and a non-negligible scatter of the data) does not allow for a unique light-curve solution. Nevertheless, we show that the assumption of normal primary-component mass and radius corresponding to the O9.5 II classification (25 $M_{\odot}$, 16–17 $R_{\odot}$) leads to consistent parameters for the system