Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids

We report the angular diameter measurements of seven classical Cepheids, X Sgr, η Aql, W Sgr, ζ Gem, β Dor, Y Oph andCar that we have obtained with the VINCI instrument, installed at ESO's VLT Interferometer (VLTI). We also present reprocessed archive data obtained with the FLUOR/IOTA instrument on ζ Gem, in order to improve the phase coverage of our observations. We obtain average limb darkened angular diameter values of θLD(X Sgr) = 1.471 ± 0.033 mas, θLD(η Aql) = 1.839 ± 0.028 mas, θLD(W Sgr) = 1.312 ± 0.029 mas, θLD(β Dor) = 1.891 ± 0.024 mas, θLD(ζ Gem) = 1.747 ± 0.061 mas, θLD(Y Oph) = 1.437 ± 0.040 mas, and θLD(� Car) = 2.988 ± 0.012 mas. For four of these stars, η Aql, W Sgr, β Dor, andCar, we detect the pulsational variation of their angular diameter. This enables us to compute directly their distances, using a modified version of the Baade-Wesselink method: d(η Aql) = 276 +55

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