All Figures

$\begin{figure} \par\includegraphics[width=13cm,clip]{aa9049f1.eps}\end{figure}$ Figure 1: Color-magnitude diagrams of the selected objects in the $\sigma$ Ori ( left panel) and $\lambda$ Ori ( right panel) clusters. In the left panel, triangles represent the probable members on the basis of low-resolution spectroscopy, circles represent the candidate members on the basis of photometry and squares represent the non-members on the basis of photometry; filled symbols mark sources with an X-ray counterpart. In the right panel, triangles represent the sources included only in the Dolan & Mathieu (1999) catalogue, circles represent the sources included only in the Barrado y Navascués et al. (2004) catalogue and squares represent the sources included in both catalogues. Continuous and dotted lines are, respectively, isochrones at 1, 2, 5, 10 and 20 Myr and tracks at 0.2, 0.3, 0.4, 0.6, 1.0 $M_{\odot }$ from the Siess et al. (2000) models, using the Kenyon & Hartmann (1995) table for the conversion of colors and magnitudes into temperatures and luminosities. Isochrones and tracks are corrected for the distance modulus (7.72 and 8.01 for $\sigma$ Ori and $\lambda$ Ori, respectively) and in the case of $\lambda$ Ori also for reddening, considering a color excess E(B-V)=0.12 (Diplas & Savage 1994) and using the Munari & Carraro (1996) transformation laws; for the $\sigma$ Ori cluster, as discussed by Oliveira et al. (2004), reddening is negligible.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{aa9049f2.eps}\end{figure}$ Figure 2: Spectra of 4 stars of the sample. The three upper rows show spectra obtained with the HR15 grating: in the left panels we show the whole spectral range, while the middle and right panels show the range around the H $\alpha$ and the Li line, respectively. For star S71, in the right panel the SII doublet at 6716 and 6731 Å can also be seen. The bottom row shows a spectrum obtained with the HR21 grating: the whole spectrum is shown in the left panel, while the middle and right panels show the range around the CaII triplet.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig3.ps}\end{figure}$ Figure 3: H $\alpha$ variability of the CTTSs as a function of the H $\alpha$ width. The upper and bottom panels show, respectively, the variability of the pEWs and of the H $\alpha$ width at 10% of the peak.
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In the text

$\begin{figure} \par\includegraphics[width=13.2cm,clip]{9049fig4.eps}\end{figure}$ Figure 4: Velocity vs. time for the 6 binary systems reported in Table 8. In the left and right panels we show the values obtained from the runs performed in October and December, respectively. The continuous and dotted curves show the sine functions that fit the data.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig5.ps}\end{figure}$ Figure 5: Radial velocity distribution of the observed sources in the two clusters. Only single stars and the binaries of Table 8 are included. The dashed curves are the functions resulting from the fit of the velocity distribution with a weighted sum of two Gaussians.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig6.ps}\end{figure}$ Figure 6: Li pEWs vs. R-I for the stars of the $\sigma$ Ori (filled circles) and $\lambda$ Ori (open triangles) cluster samples with available optical photometry.
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In the text

$\begin{figure} \par\includegraphics[width=12.5cm,clip]{9049fig7.ps}\end{figure}$ Figure 7: CMD of the observed targets in the $\sigma$ Ori ( left panel) and $\lambda$ Ori ( right panel) cluster. Filled circles and crosses indicate, respectively, members and non-members of the two clusters. Stellar magnitudes are corrected for the distance and reddening as in Fig. 1. Continuous, dashed and dot-dashed lines represent the isochrones at 1, 2, 5, 10 and 20 Myr, the Zero Age Main Sequence (ZAMS) and evolutionary tracks at 0.2, 0.3, 0.4, 0.6, 1.0 $M_{\odot }$ from the Siess et al. (2000) models, respectively.
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{9049fig8.ps}\end{figure}$ Figure 8: H $\alpha$ pEW as a function of the H $\alpha$ width at 10% of the peak measured from the summed spectra of the $\sigma$ Ori ( left panel) and $\lambda$ Ori ( right panel) cluster members. Filled and open symbols represent stars with and without a circumstellar disk according to the Spitzer data, while squares in the left panel represent stars without Spitzer photometry. Median error bars are reported on the bottom-right corner.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig9.ps}\end{figure}$ Figure 9: Age distribution of the members selected in $\sigma$ Ori and $\lambda$ Ori. Ages were derived from the R and I magnitudes using Siess et al. (2000) evolutionary models. As discussed in the text, the age distributions shown in this figure are not in agreement with the Li line pEWs shown in Fig. 6.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig3.ps}\end{figure}$	Figure 3: H $\alpha$ variability of the CTTSs as a function of the H $\alpha$ width. The upper and bottom panels show, respectively, the variability of the pEWs and of the H $\alpha$ width at 10% of the peak.
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$\begin{figure} \par\includegraphics[width=13.2cm,clip]{9049fig4.eps}\end{figure}$	Figure 4: Velocity vs. time for the 6 binary systems reported in Table 8. In the left and right panels we show the values obtained from the runs performed in October and December, respectively. The continuous and dotted curves show the sine functions that fit the data.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig5.ps}\end{figure}$	Figure 5: Radial velocity distribution of the observed sources in the two clusters. Only single stars and the binaries of Table 8 are included. The dashed curves are the functions resulting from the fit of the velocity distribution with a weighted sum of two Gaussians.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig6.ps}\end{figure}$	Figure 6: Li pEWs vs. R-I for the stars of the $\sigma$ Ori (filled circles) and $\lambda$ Ori (open triangles) cluster samples with available optical photometry.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{9049fig9.ps}\end{figure}$	Figure 9: Age distribution of the members selected in $\sigma$ Ori and $\lambda$ Ori. Ages were derived from the R and I magnitudes using Siess et al. (2000) evolutionary models. As discussed in the text, the age distributions shown in this figure are not in agreement with the Li line pEWs shown in Fig. 6.
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