  In [arXiv: 1312.1118] the author has found the formal series describing
radial adiabatic pulsations of polytropes. These series include a
time-dependent function, unknown yet. If a polytrope is uniform this function
is obtainable; it is governed by the well-known equation of Bhatnagar and
Kothari. In the present work it is shown that the pulsations of a polytrope
with a near-equilibrium stratification are described by the same equation.
These pulsations are nonlinear and their periods do not equal to the periods of
the standard model. The periods of the vanishingly small pulsations are
obtained; using them, the period-mass-radius relation is deduced. This relation
is used to estimate masses of 11 chosen Cepheids with firmly known radii; all
the masses found are between 5 and 11 solar masses that correlate well with the
results of the stellar evolution theory.
