Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESO ’ s Very Large Telescope Interferometer , we measure precisely the diameters of several symbiotic and related stars : HDÂ 352 , HDÂ 190658 , V1261Â Ori , ERÂ Del , FGÂ Ser , and AGÂ Peg .
These diameters – in the range of 0.6 to 2.3 milli-arcseconds – are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer .
We also provide the first spectroscopic orbit of ERÂ Del , based on CORAVEL and HERMES/Mercator observations .
The system is found to have an eccentric orbit with a period of 5.7 years .
In the case of the symbiotic star FG Ser , we find that the diameter is changing by 13 % over the course of 41 days , while the observations of HDÂ 352 are indicative of an elongation .
Both these stars are found to have a Roche filling factor close to 1 , as is most likely the case for HD 190658 as well , while the three other stars have factors below 0.5–0.6 .
Our observations reveal the power of interferometry for the study of interacting binary stars – the main limitation in our conclusions being the poorly known distances of the objects .