surface density which is almost constant inside a radius of $\simeq100\arcsec$.

In order to evaluate the structural parameters of M55, we have fitted the profile Figure \[profile\] with a multi-mass isotropic [@King66] model as described in the previous Section. In the following table we show the parameters of the best fitting model and we compare them with the results of [@Trager95], [@Pryor91], and [@Irwin84]:

[lccc]{} Author & $c$ & $r_c$ & $r_t$\
This paper & 0.83 & $143''$ & $970''$\
Trager   & 0.76 & $170''$ & $977''$\
Pryor   & 0.80 & $140''$ & $876''$\
Irwin and Trimble & $\sim1.0$ & $\sim120''$ & $\sim1200''$\

The concentration parameter of M55 is one of the smallest known for a globular cluster. Such a small concentration implies strong dynamical evolution and indicates that the cluster is probably in a state of high disgregation [@Aguilar88; @Gnedin97].

Our value of the tidal radius is well in agreement with that of [@Trager95] who used a similar method to fit the data. [@Pryor91] give a value of $r_t$ 10% smaller than ours. We note that Pryor and Trager used the same observational data set. The difference with [@Irwin84] is probably due to the fact that the authors have 