Roche-lobe filling factor of mass-transferring red giants: the PIONIER view H. M. J. Boffin, M. Hillen, J. P. Berger, A. Jorissen, N. Blind, J. B. Le Bouquin, J. Mikołajewska and B. Lazareff A&A, 564 (2014) A1 Published online: 26 March 2014 DOI: 10.1051/0004-6361/201323194