2012A&A...541A..87G

We report the first angular diameter measurements of two classical Cepheids, FF Aql and T Vul, that we obtain using observations with the FLUOR instrument installed at the CHARA interferometric array. We derive average limb-darkened angular diameters of θLD=0.878±0.013mas and θ_LD=0.629±0.013mas, respectively, for FF Aql and T Vul. Combining these angular diameters with the HST-FGS trigonometric parallaxes leads to linear radii R=33.6±2.2R_☉ and R=35.6±4.4R_☉, respectively. The comparison with empirical and theoretical period-radius relations leads to the conclusion that these Cepheids are pulsating in their fundamental mode. The knowledge of this pulsation mode is of prime importance to calibrating the period-luminosity relation with a uniform sample of fundamental mode Cepheids.