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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{aa09520f1.eps}\end{figure}$ Figure 1: Top window: variation in the relative spatial density of stars and brown dwarfs of spectral type i in the thin disc with the heliocentric distance from 5 to 10 000 pc in the direction of three representative young clusters: Upper Scorpius (dotted line), Pleiades (dash-dotted line), and $\sigma$ Orionis (- red - solid line). The location of the clusters are indicated by a star `` $\star$ '' symbol. Bottom window: differences between the relative spatial densities using the correct (Eq. (2)) and approximate (Eq. (3)) expressions.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{aa09520f2.eps}\end{figure}$ Figure 2: Simulated $I_{\rm SSA}$ vs. $I_{\rm SSA}-K_{\rm s, 2MASS}$ colour-magnitude diagram of a hypothetical cluster survey. (Red) Filled stars (10): hypothetical confirmed cluster members with spectroscopic signatures of youth; (blue) open stars (3): hypothetical cluster member candidates based on photometry; (blue) small dots: probable fore- and background sources; (red) thick dot-dashed line: selection criterion used for separating cluster members and candidates from probable fore- and background sources; (black) thin solid/dotted lines: ``strips'' where the number of contaminants are computed. The height of the strips were set to 1.5 mag for clarity. Small dots and open stars are actual data from a multiband ( $BRI_{\rm SSA}JHK_{\rm s, 2MASS}$ ) 90 arcmin-radius survey centred on HD 221356 (aka Kö 3 A), an F8V star with an M-L-type binary companion in a very wide orbit (Gizis et al. 2000; Caballero 2007). The mean sequence of the hypothetical cluster is parallel to the thick dot-dashed line, shifted 0.5 mag to the red. Filled stars were generated randomly by adding a 0.5 mag-amplitude noise to the mean cluster sequence (i.e. the selection criterion is $\sim$ 1 $\sigma$ shifted to the blue with respect to the mean cluster sequence). (SSA: SuperCOSMOS Science Archive, Hambly et al. 2001; 2MASS: Two-Micron All Sky Survey, Skrutskie et al. 2006).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{aa09520f3.eps}\end{figure}$ Figure 3: Local spatial density of ultracool dwarfs as a function of spectral type from the data in Table 3.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{aa09520f4.eps}\end{figure}$ Figure 4: I vs. I-J colour-magnitude diagram of the $\sigma$ Orionis cluster. Large filled circles with error bars: cluster members and candidates (the faint red outlier is the T-type object S Ori 70); solid and dashed lines: COND03 and DUSTY00 3 Ma-old isochrones, respectively, shifted to d = 385 pc (Chabrier et al. 2000; Baraffe et al. 2003); dotted lines: dwarf sequence in Table 3 shifted by distance moduli m-M = 3.0, 6.0, 9.0, 12.0, and 15.0 mag, from top to bottom. The faintest data point of the COND03 theoretical isochrone ( $I-J \approx 3.0$ mag, $I \approx 29.0$ mag) corresponds to an 1.0 $M_{\rm Jup}$ -mass object. L and T field dwarfs have approximate colours redder than $I-J \approx 3.5$ and 4.5 mag.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{aa09520f3.eps}\end{figure}$	Figure 3: Local spatial density of ultracool dwarfs as a function of spectral type from the data in Table 3.
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