V446 Cephei: a β Cep pulsator in a multiple system

ABSTRACT $\beta$ Cep stars in eclipsing binary (EB) systems give us an opportunity to put observational constraints on their structure and stellar parameters. We present a comprehensive analysis of the $\beta$ Cep star in the EB V446 Cep using TESS photometry and HERMES spectra. We calculate the stellar and orbital parameters using light curve modelling and spectral disentangling. The EB has an orbital period of $3.808567 \pm 0.000012$ d and a mass ratio of $0.1550 \pm 0.0012$. We find the $\beta$ Cep star to have a mass of $10.68 \pm 0.06$ $\mathrm{{\rm M}_{\odot }}$, a radius of $5.864 \pm 0.033$ $\mathrm{R}_{\odot }$, and a $T_{\rm eff}$ of $24220 \pm 180$ K. The secondary has a mass of $1.657 \pm 0.017$ $\mathrm{{\rm M}_{\odot }}$, a radius of $1.530 \pm 0.014$ $\mathrm{R}_{\odot }$, and a $T_{\rm eff}$ of $9080 \pm 390$ K. We also extract the abundances of C, N, O, Mg, and Si for the $\beta$ Cep star, which are found to be consistent with galactic OB binaries. We identified 21 distinct pulsation frequencies, with the dominant mode at 10.24324 d$^{-1}$, which corresponds to a near-harmonic of the system’s orbital frequency. The two stars in the EB have asynchronous rotation, with both stars rotating faster than the orbital frequency. We detect a companion to the EB using eclipse timing variations and period changes of the dominant pulsation frequency. We calculate the minimum mass of this tertiary companion to be $4.11 \pm 0.32$ $\mathrm{{\rm M}_{\odot }}$ which is on an orbit of 2303$\pm$69 d around the EB. Using spectral energy distributions and theoretical isochrones, we conclude that V446 Cep is either a co-evolving hierarchical 2 + 2 quadruple or a triple system where the third body is a compact object.