  Symbiotic stars display absorption lines of a cool red giant together with
emission lines of a nebula ionized by a hotter star, indicative of an active
binary star system in which mass transfer is occurring. PIONIER at the VLT has
been used to combine the light of four telescopes at a time to study in
unprecedented detail how mass is transferred in symbiotic stars. The results of
a mini-survey of symbiotic stars with PIONIER are summarised and some tentative
general results about the role of Roche lobe overflow are presented.
