Period analysis of two non-Blazhko RRab stars, FN Lyr and V894 Cyg, based on Kepler photometry: evidence of low-mass companions on wider orbits

Abstract

Long-cadence-corrected pre-search data conditioning (PDC) fluxes of FN Lyr (KIC 6936115) and V894 Cyg (KIC 9591503), observed continuously by the Kepler mission, spanning over 1470 d, are used to determine hundreds of times of maximum and minimum for the analysis of O - C residuals. The interpretation of the clear variations in the O - C diagrams is that these are caused by the light-travel-time effect as a result of additional companions. The mass functions of the companions are f (M) = (3.94 +/- 0.82) x 10(-6) and (2.01 +/- 0.22) x 10(-4) M-circle dot. Assuming that the orbital plane inclination follows a random distribution, the companions to both stars can be constrained to be substellar objects (brown dwarf or giant planet), with 89.4 and 59.4 per cent probability, respectively. Under the assumption that the orbital inclination equals 90 degrees, the distances between the companions and the central RR Lyrae stars at periastron should be 1.03 and 0.50 au, respectively. In addition, the orbital periods are 794.8 and 1084.4 d for FN Lyr and V894 Cyg, respectively. By comparing these orbital parameters with those of B subdwarf stars in binary systems, there are strong hints that horizontal branch stars might have different evolution histories. The long-term pulsation period changes are also discussed. Based on our studies, RR Lyrae stars in binary systems are not rare, at least among binary systems with wider separations.