total contribution of the measured field stars is larger, but it is still less than $25\%$ of the cluster stars (the worst case applies to the faintest magnitude bin): this means that the possible M55 halo star over-subtraction in the field-corrected LF is always less than $6\%$ ($25\% \times 25\%$), negligible for our purposes.

Mass function of M55 \[Sm55mf\]
-------------------------------

In order to build a mass function for the stars of M55, we needed to adopt a distance modulus and an extinction coefficient. [@Shade88] give $(m-M)_V=14.10$, E$(B-V)=0.14\pm0.02$, while, more recently, [@Mand96] give $(m-M)_V=13.90\pm0.07$, E$(B-V)=0.14\pm0.02$. In the absence of an independent measure made by us, we adopted the values published by [@Mand96] because they are based on the application, with updated data, of the subdwarfs fitting method. Using the LFs of the previous Section we build the corresponding mass functions using the mass-luminosity relation tabulated by [@VDB85] for an isochrone of $Z=3\times10^{-4}$ and an age of 16 Gyr [@Alcaino92]. The MFs for the three radial intervals are presented in Figure \[M55mf\]. The MFs are vertically shifted in order to make their comparison more clear.

The MFs are significantly different: the slopes of the MFs increase moving outwards as expected from the 