The cluster-type variables deserve particular attention since they are connected with two problems of fundamental importance: the phenomenon of pulsating stars and the existence of different subsystems or populations of stars in our own and other galaxies. On the other hand there are some obscure points in our understanding of the physical and dynamical properties of this group of stars as, for instance, their failure to give a period-spectrum relation or to obey the proper period-density relation. With this in view Dr Struve suggested to me during my stay at the Yerkes and McDonald Observatories to undertake a comparative spectrophotometric investigation of a series of these stars.