(O−C) phase shifts and amplitude variations in the y passband of 10 dominant modes from 2005 to 2012. For each mode, the phase shifts (indicating frequency variations) were determined from an average frequency. The formal uncertainties are generally of the size of the plotted dots and therefore not shown. The solid lines represent least-squares fits of constant frequency changes. The excellent fits for most modes indicate constant frequency (period) changes, suggesting a timescale of at least decades.