SIMBAD references

In order to investigate the period variations of AHB1 pulsating variables due to evolutionary effects predicted by Sandage, Diethelm and Tammann (1994), the photographic Sky Survey of the Sonneberg Observatory, Germany, for the years 1928-1994 was searched. By constructing seasonal mean light curves and using the rise-time method, it is shown that the periods of four of six field AHB1 variables examined (EK Del, UY Eri, CE Her, V716 Oph) have been increasing over the last 70 years, while two stars, BF Ser and XX Vir, shows no change of period in this interval. The rate of the period change is in good agreement with the theoretically expected values.