ON THE CHANGES OF PERIODS OF LONG PERIOD CEPHEIDS by P. P. PARENAGO, Moscow The changes oŁ periods of 43 long period cepheids were studied. Period variations were found for 35 stars, 25 of them belonging to the flat component of the Galaxy and 10 to the spherical one. The periods do not change continuously, but always abruptly: the period P which existed during a certain interval of time Delta E changes to another period P + Delta P. The number n of different periods covered by observations varies for different stars between 2 to 12. For n = 2 there is a predominance of positive values of Delta P (for 13 of 18), but for n > 2 this predominance vanishes (32 of 63). That means that both signes have the same probability. For n > 2 there is more often a successive change of positive and negative values of Delta P. The value (abs(mean(Delta P)/P)) increases to two orders and Delta E decreases on two orders for periods ranging from 4 to 45 days. Therefore the quantity has about the same value for all the Cepheids. This value is about 0.1 for Cepheids of the flat component and 0.6 for Cepheids of the spherical component of the Galaxy. That is, the latter stars change their periods more often and stronger than the first ones. The undimensional quantity given above is something like a new characteristic of Cepheids. Some short periodical Cepheids follow probably the same rule. The cause of the period changes of Cepheids may be very small variations of their physical characteristics (from 0.000004m to 0.004m in absolute magnitude and from 0.005 deg to 5 deg in the surface temperature), which presently could not be detected by direct measurements. * Expressed in the units of the period.