The dark matter content of early,- type galaxies (ETGs) is a hotly debated topic with contrasting results arguing in favour or against the presence of significant dark mass within the effective radius and the change with luminosity and mass. In order to address this question, we investigate here the global mass - to - light ratio $Upsilon(r) = M(r)/L(r)$ of a sample of 21 lenses observed within the Sloan Lens ACS (SLACS) survey. We follow the usual approach of modeling the galaxy as a two component systems, but we use a phenomenological ansatz for $Upsilon(r)$, proposed by some of us in Tortora et al. (2007), able to smoothly interpolate between constant $M/L$ models and a wide class of dark matter haloes. The resulting galaxy model is then fitted to the data on the Einstein radius and velocity dispersion. Our phenomenological model turns out to be in well agreement with the data suggesting the presence of massive dark matter haloes in order to explain the lensing and dynamics properties of the SLACS lenses. According to the values of the dark matter mass fraction, we argue that the halo may play a significant role in the inner regions probed by the data, but such a conclusion strongly depends on the adopted initial mass function of the stellar population. Finally, we find that the dark matter mass fraction within $R_{eff}$ scales with both the total luminosity and stellar mass in such a way that more luminous (and hence more massive) galaxies have a larger dark matter content.
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