Period changes in W UMa-type eclipsing binaries: DK Cygni, V401 Cygni, AD Phoenicis and Y SextantisM. Wolf, P. Molík, K. Hornoch and L. ŠarounováAstron. Astrophys. Suppl. Ser., 147 2 (2000) 243-249DOI: https://doi.org/10.1051/aas:2000300