Towards early-type eclipsing binaries as extragalactic milestones: III. Physical properties of the O-type eclipsing binary OGLE LMC-ECL-21568 in a quadruple system

We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period PA=3.2254367 days, that forms part of a higher-order multiple system in a configuration (A+B)+C. We derived masses of the Aa+Ab binary M₁= 19.02 ± 0.12 \,M_, M₂= 17.50 ± 0.13 \,M_, radii R₁= 7.70 ± 0.05 \,R_, R₂= 6.64 ± 0.06 \,R_, and temperatures T₁ = 34250 ± 500 K, T₂ = 33750 ± 500 K. From the analysis of radial velocities, we found a spectroscopic orbit of A in the outer A+B system with PA+B=195.8 days (PA+B/PA≈ 61). In the O-C analysis, we confirmed this orbit and found another component orbiting the A+B system with PAB+C=2550 days (PAB+C\,/PA+B≈ 13). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, MB 10.7 M_. From the light-travel time effect fit to the O-C data, we obtained the limit for the mass of the fourth component, MC 7.3 M_. These extra components contribute to about 20% to 30% (increasing with wavelength) of the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 ± 0.06 mag, a reddening of 0.16 ± 0.02 mag, and an RV of $3.2$. This work is part of our ongoing project, which aims to calibrate the surface brightness-color relation for early-type stars.