MAGIC's interferometric diameters of $θ$ Ophiuchi and $β$ CMa and comparison with asteroseismic modelling
$β$ Cephei type stars are massive ($8-25$M$_{\odot}$) and known to have several pulsation modes with periods typically of a few hours. Despite a better understanding of their physics since 1992, the inner structure of these stars involves key stellar physics processes that are still uncertain. Asteroseismology offers insights into these variable stars by analysing pulsation modes, helping to probe their inner layers and understand their structure and mixing processes. We measure the angular diameter of two $β$ Cephei stars, $θ$ Oph and $β$ CMa, using the MAGIC Stellar Intensity Interferometer and compare them, together with the Gaia and Hipparcos parallaxes, to the predictions of the asteroseismic model. The latter are based on Levenberg-Marquardt local minimisation of stellar evolution model parameters to fit observed pulsation modes. In the case of $θ$ Oph, the observed angular diameters are compatible with the asteroseismic model. For $β$ CMa, the radius derived from the angular diameter and the Hipparcos parallax is in tension, at the $2.5σ$ level, with the asteroseismic prediction. We show that this tension mirrors a pre-existing conflict between independent spectroscopic determinations of the surface gravity of the star, that percent-level systematics of the stellar models are quantitatively insufficient to explain it, and that it most likely points to a bias of the Hipparcos parallax. In the frame of this paper, the statistical uncertainty on the interferometric results is similar to the systematic uncertainty on the parallaxes. With the already implemented addition in early 2024 of the first Large-Sized Telescope (CTAO-North LST-1) to the setup, the uncertainty in the angular diameter of these stars is expected to decrease by an order of magnitude, allowing to further constrain stellar evolution models and parallaxes.