Period evolution (O–C) diagrams for U Sgr (dP/dt =  + 0.073 s yr^-1) and CF Cas (dP/dt =  −0.072 s yr^-1) facilitate the selection of an isochrone (Fig. 2). The new results imply that U Sgr is traversing the instability strip for the third time and is evolving toward the cool-end of the instability strip, whereas CF Cas may be a second crosser venturing toward hotter temperatures (the scatter is admittedly large). The lower panels display the residuals from the polynomial fits applied. Sterken (2005) relays comprehensive information regarding period evolution (O–C) diagrams.