1: Observatoire de la Côte d'Azur, Département Fresnel, Caussols,
06460 Saint Vallier de Thiey
2: Observatoire Midi Pyrénées, 14 Av. Ed. Belin, 31100 Toulouse
3: Collège de France & Observatoire de la Côte d'Azur, Caussols, 06460 Saint Vallier de Thiey
4: Observatoire de Lyon, 69651 Saint Genis Laval Cedex
* Present address: Observatoire de Haute Provence, 04870 St Michel l'Observatoire
(Submitted to Astronomy and Astrophysics)
Delta Cephei is a pulsating star which fades every 5 days, and gave its name to the cepheid class of variable stars. Because their intrinsic luminosity is a function of their period, these are classicaly used to determine galactic and extragalactic distances. Theory and spectroscopic evidences have suggested fluctuations of the star's diameter, in synchronism with the photometric modulation, but these fluctuations could not be resolved angularly with even the largest telescopes. We have detected such size variations with the "Grand Interféromètre à deux Télescopes". The combined spectroscopic and interferometric data improve the distance determination for d Cephei, thus providing a more accurate distance scale to other cepheids and their host galaxies.