  We compared period changes derived from O-C diagrams for 63 classical
Cepheids from our Galaxy with model calculations. We found that for Cepheids
with log P > 1.0 the observed changes are smaller than predicted values, except
variable SZ Cas. However some of the first overtone Cepheids, particularly EU
Tau and Polaris, change its period much faster than it follows from theory.
Summary of the known data on the period changes in Cepheids from the Galaxy and
from the Magellanic Clouds (previous papers) leads to conclusion that none of
the 999 Cepheid is undergoing the first crossing of the instability strip. Also
the observed period changes for long-period Cepheids are a few times slower
than predicted by the models. These results imply that much larger fraction of
helium is burnt in the Cepheid stage than it is predicted by models.
