The variation of the mass-to-light ratios M/L of early type galaxies as
function of their luminosities L is investigated. It is shown that the tilt
beta=0.27 (in the B--band) of the fundamental plane relation M/L ~ L^{beta} can
be understood as a combination of two effects: about one-quarter (i.e. dbeta
=0.07) is a result of systematic variations of the stellar population
properties with increasing luminosity. The remaining three-quarters (i.e. dbeta
=0.2) can be completely attributed to nonhomology effects that lead to a
systematic change of the surface brightness profiles with increasing
luminosity. Consequently, the observed tilt in the K-band (beta=0.17) where
stellar population effects are negligible, is explained by nonhomology effects
alone. After correcting for nonhomology, the mean value of the mass-to-light
ratio of elliptical galaxies (M/L_B) is 7.1+-2.8 (1 sigma scatter).Comment: 8 pages, 3 figures, ApJL, 600, 39, minor changes made to match the
published versio