Binarity and beyond in A stars II. Disentangling the four stars in the vicinity of the triple HIP 87813 within the quintuple system HJ2814

A-type stars are the progenitors of about half of the white dwarfs (WDs) that currently exist. The observational mappings of both multiplicities are far from complete and their connection is not known. We are in the process of obtaining tight constraints on a sample of 108 southern and nearby A-stars that are part of the VAST sample by conducting near-infrared (NIR) interferometric observations to the stars among them which have large Gaia-Hipparcos accelerations. In this paper, we combine spectroscopy, adaptive optics imaging, NIR interferometry, and Gaia-Hipparcos astrometry in order to disentangle the stars in the complicated HIP 87813 = HJ2814A system. We show that (i) a previously discovered faint star separated by 2 arcsec is actually a background source; (ii) the Gaia-Hipparcos acceleration is caused by a newly discovered M ∼0.77 M_⊙ star that was missed in previous AO images and we solve for its a = 27 au (P ∼70 yrs) astrometric orbit; (iii) by combining archival spectra with interferometric observations, we show that the A star has a very close M ∼0.85 M_⊙ companion on a a = 0.16 au (P = 13.4 d) orbit. The mutual inclination in the hierarchical triple is constrained to φ_rel =46 ± 15^° but Kozai-Lidov eccentricity oscillations in the inner binary are currently very small and damped by general relativistic precession. The system HJ2814 is one of only about fifteen known 5+ multiples with an A-star primary, and will result in a system of at least two WDs within about a Hubble time.