We predict from a survey of equations of state and observations of X–ray pulsations from SAX J1808.4–3658 , that the upper limit of the mass of the compact star is 2.27 M _ { \odot } .
The corresponding upper limit of the radius comes out to be 9.73 km .
We also do a study to estimate the lower limit of the mass of the compact star .
Such a limit is very useful to put constraints on equations of state .
We also discuss the implications of the upper mass limit for the detection of the source in radio frequencies .
We point out that the possible observation of radio–eclipse may be able to rule out several soft equation of state models , by setting a moderately high value for the lower limit of inclination angle .