Observed locations of the classical Cepheid (upper black circle) and its stable red giant companion (lower black circle) on the luminosity-effective temperature diagram. Blue and red lines show the blue and red edges of the Cepheid instability strip, for metallicities of Z = 0.008 (solid lines), and Z = 0.004 (dashed lines). Both stars are, within the observational uncertainties, located inside the theoretical instability strip (Bono et al. 2005), for both metallicities. The blue and red pointed lines show the evolutionary tracks for the Cepheid, and its slightly less massive companion, respectively. The solid black line is the isochrone computed for an age of 205 Myr which fits the positions of both stars, within the uncertainties (for more details on the calculations, see the text).