NSVS 2569022: A peculiar binary among W UMa stars with extremely small mass ratios

Abstract

Photometric observations of the W UMa binary NSVS 2569022 are presented. The light curve solution reveals that both components are of F spectral type (temperatures T 1 = T 2 = 6100 K). NSVS 2569022 undergoes a total eclipse of W subtype and the mass ratio is well-determined. Its extremely small value of only 0.077 implies that the target will probably experience instability and a possible merger. This value ranks NSVS 2569022 in sixth place among binaries with the smallest mass ratio. Based on an empirical relation of "period - total mass" for low mass-ratio binaries, we estimate the global parameters of NSVS 2569022: masses M 1 = 1.17 M o and M 2 = 0.09 M o; radii R 1 = 1.19 R o and R 2 = 0.38 R o; luminosities L 1 = 1.73 L o and L 2 = 0.17 L o. An analysis of the characteristics of binaries with extremely low-mass ratios is made. NSVS 2569022 turns out to be a peculiar binary among W UMa stars with extremely small mass ratios due to its unexpectedly small fill-out factor of only 0.014 (slightly overcontact configuration).