Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S2 / 1 / 2

Eclipses justify an approximately edge-on orbit, and equal velocity amplitudes imply equal masses. The relative speed is 200 km s${}^{-1}$, so $a=(v_1+v_2)P/(2\pi )\simeq 1.50\times 10^{10}$ m, or $\boxed{a\simeq 0.100\mathrm{AU}}$. Substitution in the binary period law gives $M_1+M_2\simeq 4.52M_{\odot }$, hence $\boxed{M_1=M_2\simeq 2.26M_{\odot }}$. Inclination corrections scale the dynamical masses as ${\sin }^{-3}i$ if exact edge-on viewing is relaxed.