The projection factor of delta Cephei A calibration of the   Baade-Wesselink method using the CHARA Array
  Cepheids play a key role in astronomy as standard candles for measuring intergalactic distances. Their distance is usually inferred from the Period-Luminosity relationship, calibrated using the semi-empirical Baade-Wesselink method. Using this method, the distance is known to a multiplicative factor, called the projection factor. Presently, this factor is computed using numerical models - it has hitherto never been measured directly. Based on our new interferometric measurements obtained with the CHARA Array and the already published parallax, we present a geometrical measurement of the projection factor of a Cepheid, delta Cep. The value we determined, p = 1.27$\pm$0.06, confirms the generally adopted value of p = 1.36 within 1.5 sigmas. Our value is in line with recent theoretical predictions of Nardetto et al. (2004). 
M\'erand Antoine (LESIA), Pierre Kervella (LESIA), Vincent Coude Du   Foresto (LESIA), Stephen T. Ridgway (LESIA, NOAO, CHAR...