The Short periods of Delta Scuti Stars allow the observational detection of the period changes expected from the stellar evolution within several tens years. For about 30 years we are keeping this topic as a small systematic observational program with our 60 cm telescope. Here we publish the period variation of 18 stars in table I. The data of 28 And are taken from R. Garrido et al. in AAp 144 (1983), 211; the period variation rate of 4 CVn is given by M. Breger. Both of them are low amplitude variables.

From these data we find out that both the population I and II variables can have period increasing and decreasing. The rate is between 2 × 10−6 to 8 × 10−8 days/year. Due to the period range of about 0.2 days, the time scale of period variation in one direction is limited within about 2 million to 200 million years. Usually we take the mass of this A to F type variables as 1.5 to 2.0 solar mass, so the main sequence life time is about several billion years. If all these A to F type main sequence stars will be variable in some period of its life time the possibility for a star to show Delta Scuti type variation is about hundredth, especially for the high amplitude type. Up to now we have found some 50 high amplitude variables with V < 10.5, and the total number of A3 to F5 with V < 10.5 is about 1.2 × 105, so the incidence of high amplitude variables is about 0.04%.