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tokens: This study is based on observations obtained in Rozhen Observatory, European Southern Observatory program 074.D-0114 and Asiago Observatory. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This research has made use of the NASA's Astrophysics Data System, and the SIMBAD astronomical data base, operated by CDS at Strasbourg, France. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non- HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. We are thankful to U. Munari who made available the Asiago spectra of AG Peg obtained in 2002. We are grateful to the anonymous referee for valuable comments and suggestions. TVT is very grateful to Svet Zhekov for the many useful discussions and Sz. Zywica for some help with English.