Putting their chemically peculiarities aside, Ap and Am stars have many common properties. The discriminating property between them could be the rotation of their interiors beneath their slowly rotating surface. In the PV Cas (HD 240208) binary system whose light curve shows Ap-like variation, the agreement between the theoretical and observed apsidal advance rate is satisfied only with differentially rotating models of the component stars in which rapid rotation is extended almost to the surface. Thus, it seems that there is a steep rotation rate gradient near the surface of the Ap stars. The conclusion we reach in the analysis of PV Cas system leads us to introduce the effective mass as a new conceptual tool in stellar astrophysics: $M_{\rm eff}= M_\star (1-\bar{\Lambda})^{1.75}$, where $M_\star$ and $\bar{\Lambda}$ are mass of the star and average of ratio of centrifugal to the gravitational acceleration throughout the model, respectively. We find that the effective mass of PV Cas A whose true mass is $2.82\pm 0.05\,M_\odot$ is $2.6\,M_\odot$ for both solar and metal rich compositions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html