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HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4association, previously noted to display both high-frequency radial velocity(RV) variability and magnetospheric H$\alpha$ emission. We have analysed theTESS light curve, and find that it is a $\beta$ Cep pulsator with 11 detectablefrequencies, 4 of which are independent $p$-modes. The strongest frequency isalso detectable in RVs from ground-based high-resolution spectroscopy. RVs alsoshow a long-term variation, suggestive of orbital motion with a period of$\sim$years; significant differences in the frequencies determined from TESSand RV datasets are consistent with a light-time effect from orbital motion.Close examination of the star's spectrum reveals the presence of aspectroscopic companion, however as its RV is not variable it cannot beresponsible for the orbital motion and we therefore infer that the system is ahierarchical triple with a so-far undetected third star. Reanalysis of LSDprofiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprisingpresence of a strong magnetic field in the companion star, with $\langle B_z\rangle$ about $+1.5$ kG as compared to $\langle B_z \rangle \sim -0.8$ kG forthe primary. HD 156424 is thus the second hot binary with two magnetic stars.We are unable to identify a rotational period for HD 156424A. Themagnetospheric H$\alpha$ emission appears to originate around HD 156424B. UsingH$\alpha$, as well as other variable spectral lines, we determine a period ofabout 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hotstars.

MOBSTER -- V: Discovery of a magnetic companion star to the magnetic $\beta$ Cep pulsator HD 156424