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HR 6819 is a bright ($V=5.36$), blue star recently proposed to be a triplecontaining a detached black hole (BH). We show that the system is a binary anddoes not contain a BH. Using spectral decomposition, we disentangle theobserved composite spectra into two components: a rapidly rotating Be star anda slowly rotating B star with low surface gravity $(\log g \approx 2.75)$. Bothstars show periodic radial velocity (RV) variability, but the RV semi-amplitudeof the B star's orbit is $K_{\rm B}= (62.7 \pm 1)\,\rm km\,s^{-1}$, while thatof the Be star is only $K_{\rm Be} = (4.5\pm 2)\,\rm km\,s^{-1}$. This impliesthat the B star is less massive by at least a factor of 10. The surfaceabundances of the B star bear imprints of CNO burning. We argue that the B staris a bloated, recently stripped helium star with mass $\approx 0.5\,M_{\odot}$that is currently contracting to become a hot subdwarf. The orbital motion ofthe Be star obviates the need for a BH to explain the B star's motion. Astripped-star model reproduces the observed luminosity of the system, while anormal star with the B star's temperature and gravity would be more than 10times too luminous. HR 6819 and the binary LB-1 probably formed through similarchannels. We use MESA models to investigate their evolutionary history, findingthat they likely formed from intermediate-mass ($3-7\,M_{\odot}$) primariesstripped by slightly lower-mass secondaries and are progenitors to Be + sdOBbinaries such as $\phi$ Persei. The lifetime of their current evolutionaryphase is on average $2\times 10^5$ years, of order half a percent of the totallifetime of the Be phase. This implies that many Be stars have hot subdwarf andwhite dwarf companions, and that a substantial fraction ($20-100\%$) of fieldBe stars form through accretion of material from a binary companion.

A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1