On the interpretation of the long-term cyclic period variations in RR Lyrae stars

Many RR Lyrae stars show long-term variations of their pulsation period, some of them in a cyclic way. Such behaviour can be attributed to the light-travel time effect (LTTE) caused by an unseen companion. Solutions of the LTTE often suggest very eccentric orbits and minimal mass of the companion on the order of several solar masses, thus, in the black hole range. We discuss the possibility of the occurrence of the RR Lyr-black hole pairs and on the case of Z CVn demonstrate that the LTTE hypothesis can be false in some of the binary candidates.