A study of the contact binary system ER Cephei

Photoelectric V and B light curves of ER Cephei, a 16-mag W Ursae Majoris-type binary system in NGC 188, are described. The available data are not sufficient definitely to determine whether the light curve of the system is of A type or W type, although this analysis suggests the latter. Light-curve synthesis solutions are calculated for both cases, together with their uncertainties. The results show that ER Cep is similar to field contact binaries, is in marginal contact, has a well-determined inclination angle, $$i = 79 \pm 3^\circ$$, and has a poorly determined mass ratio, $$q = 0.55 \pm 0.20$$. A brief discussion is given of system parameters for ER Cep derived from cluster membership considerations. The need for spectroscopic data and the importance of ER Cep for cluster Hertzsprung–Russell diagram gap theory is stressed.