2026MNRAS.545f1975C

$\theta ^1$ Ori E is a very young and relatively massive pre-main sequence (PMS) spectroscopic and eclipsing binary with nearly identical components. We analyse Échelle spectra of the system obtained over fifteen years and report 91 radial velocities measured from cross-correlating the observations with a suitable synthetic spectrum. The spectra of individual binary components are indistinguishable from each other, with a composite spectral type around G4 III. The projected equatorial velocity is estimated to be $v\, \mathrm{ sin}\, i=32\pm 3~\mathrm{km~s}^{-1}$, consistent with rotational synchronization. We find that the circular orbit has $P_{\rm orb} = 9.89522 \pm 0.00003\,\mathrm{ d}$, $K_1 = 83.36 \pm 0.29~\mathrm{km~s}^{-1}$, $K_2 = 84.57 \pm 0.28~\mathrm{km~s}^{-1}$, and $a\mathrm{ sin}i = 32.84\pm 0.08\ \mathrm{R}_\odot$. The mass ratio is $q = 0.9856 \pm 0.0047$, indicating nearly identical but significantly different masses. The systemic velocity of the binary, $\gamma = 29.7 \pm 0.2~\mathrm{km~s}^{-1}$, is similar to that of other Trapezium members. Using Spitzer light curves and our results, we derive $M_1 = 2.755\pm 0.043\ \mathrm{M}_{\odot }$, $M_2 = 2.720\pm 0.043\ \mathrm{M}_{\odot }$, $R_1=6.26\pm 0.31\, \mathrm{R}_{\odot }$, and $R_2=6.25\pm 0.30\, \mathrm{R}_{\odot }$. Together with our estimate of the effective temperature, $T_{\mathrm{ eff}}=5150\pm 200\ K$, a bolometric luminosity of $28.8\pm 4.6\ \mathrm{L}_{\odot }$ is derived for each component. Compared to evolutionary models of PMS stars, the binary age turns out to be less than or equal to ${\sim} 10^5$ yr. Its components are probably the most massive stars known with masses determined with precision better than 2 per cent, with both being PMS stars.