Long-term photometric behaviour of XZ Dra. Binarity or magnetic cycle of a Blazhko-type RRab star

The extended photometry available for XZ Dra, a Blazhko-type RR Lyrae star, makes it possible to study its long-term behavior. It is shown that its pulsation period exhibits cyclic, but not strictly regular variations with ~7200 d period. The Blazhko period (~76 d) seems to follow the observed period changes of the fundamental mode pulsation with dP_B/dP₀=7.7x 10⁴ gradient. A binary model cannot explain this order of period change of the Blazhko modulation, nevertheless it can be brought into agreement with the O-C data of the pulsation. The possibility of occurrence of a magnetic cycle is raised.