Predictions based on the evolutionary synthesis code PéGASE 2 on the variation of the ℳ/ℒ ratio and δ_io as a function of galaxy age. Upper panel: logarithm of the ℳ/ℒ ratio as a function of time when assuming continuous star formation at a constant SFR (SFH1), and an exponentially decreasing SFR with an e-folding time τ of 0.5 Gyr and 1 Gyr (SFH2 and SFH3, respectively). Predictions for the B and R band (blue and red, respectively) are shown with symbols (squares, solid circles, and open circles for SFH1, SFH2, and SFH3, respectively) and those for the K band with curves (blue, orange, and black for SFH1, SFH2, and SFH3, respectively). Lower panel: evolution of δ_io, as approximated by Eq. (2), when assuming SFH2 and SFH3 for the bulge (solid and open symbols, respectively) with predictions for the B, R, and K band shown in blue, red, and black, respectively.