must be real.

\[M55mf\]

Another source of error in the LF construction is represented by the LF of the field stars. As will be shown in Section \[M55rprof\], M55 has a halo of probably unbound cluster stars. The field star LF constructed from the star counts just outside the cluster can be affected by some contamination of the cluster halo. The consequence is that we might over-subtract stars when subtracting the field LF from the cluster LF, modifying in this way the slope of the mass function (the more affected magnitudes are the faintest ones). To test this possibility, we have extracted background LFs in two different anulii outside the cluster (in terms of $r_t$, $1.0<r\le1.3$ and $r>1.3$). Comparing the two background/foreground LFs we found that the number of stars probably belonging to the cluster but outside the tidal radius must be less than $\sim25\%$ of the adopted field stars in the worst case (the faintest bins). The possible over-subtraction is not a problem for the inner and intermediate LFs, where the number of field stars (after rescaling for the covered area) is always less than $\sim3\%$ of the stars counted in each magnitude bin. For the outer LF, the 