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We present a theoretical investigation of the light, radius and radialvelocity variations of the prototype $\delta$ Cephei. We find that the best fitmodel accounts for luminosity and velocity amplitudes with an accuracy betterthan $0.8\sigma$, and for the radius amplitude with an accuracy of $1.7\sigma$.The chemical composition of this model suggests a decrease in both helium (0.26vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover,distance determinations based on the fit of light curves agree at the$0.8\sigma$ level with the trigonometric parallax measured by the Hubble SpaceTelescope (HST). On the other hand, distance determinations based on angulardiameter variations, that are independent of interstellar extinction and of the$p$-factor value, indicate an increase of the order of 5% in the HST parallax.

Theoretical fits of the \delta Cephei light, radius and radial velocity curves