1 Introduction

W UMa-type eclipsing binaries are an important source of information about the evolution of close binary systems. In this paper, we report results of our observations monitoring W UMa-type eclipsing binaries with period changes (Wolf et al. 1996; Molík & Wolf 1998). In particular, four objects with the similar brightness ( $V \simeq 10$ mag), namely DK Cyg, V401 Cyg, AD Phe and Y Sex, are analysed here. With the exception of DK Cyg, which was found to be a variable star by Guthnick & Prager (1927), the other variables were discovered by Cuno Hoffmeister. They all are well-known close binaries and their long epoch of observations gives us the possibility to study many interesting phenomena in these systems. The purpose of the present work is to present new photoelectric times of minimum light, to make a period study of these systems and to derive new light elements.