Revisiting symbiotic binaries with interferometry

Symbiotic stars, which generally comprise a red giant and an accreting white dwarf, are excellent laboratories to understand mass transfer in long-period binaries. One of the fundamental questions is how mass is transferred from the red giant to the white dwarf. We used interferometric measurements made with the VLTI/PIONIER instrument, combined with Gaia data, to measure the radius of the giant in seven symbiotic systems. We further placed the giants in the Hertzsprung-Russell diagram, allowing us to estimate their mass and to show that they are all very evolved and likely on the asymptotic giant branch. We compared our measured giant radii to their Roche lobe radius, showing that, except for ZZ CMi, all giants are well within their Roche lobe and that mass transfer likely takes place via stellar wind. Our interferometric data provide further evidence that the giant in ZZ CMi (nearly) fills its Roche lobe. Our conclusions are still hampered by the poor characterisation of some of the giants or their binary orbits, and we encourage the community to make an effort to provide these.