space by adding CFHTLS-T0003 cosmic shear data to each of the Monte Carlo Markov chains defined in Sect. 2.2.2, aiming to determine the gain achieved by including lensing data.

The main independent piece of information provided by cosmic shear is the power spectrum on small scales. As discussed above, it has a distinctive scaling with both neutrino and CDM densities, while these two parameters for CMB are instead weakly correlated, as seen in Fig. \[fig:joint\] (left panel, largest contour). This allows us to obtain a narrow, joint constraint, approximatively in the same direction found for cosmic shear alone in Fig. \[fig:wlcont\] (middle panel). The direction orthogonal to the contour indicates the combination of parameters to which cosmic shear is mostly sensitive to. It is well approximated by the linear relation $\omega_{\mathrm{cdm}}- \omega_\nu$ and the WMAP5 + CFHTLS constraint on this combination of parameters is, $$\omega_{\mathrm{cdm}}- \omega_\nu = 0.106 \pm 0.006\; (1\sigma).$$ The figure-of-merit (FoM) quantifies the error ellipses of correlated parameters and was introduced for the parameters of the dark energy equation of state [@detf]. It is defined as the inverse area of the ellipse approximating the $p\%$ confidence level contour, centered on the contour centroid, i.e., $$\mathrm{FoM}^{-1}=\Delta\chi^2(p)\pi\,(\sigma_{11}\sigma_{22}-\sigma_{12}^2)^{1/2},$$ where $\sigma_{ii}$ 