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

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 _⊙ and M ₂ = 0.09 M _⊙; radii R ₁ = 1.19 R _⊙ and R ₂ = 0.38 R _⊙; luminosities L ₁ = 1.73 L _⊙ and L ₂ = 0.17 L _⊙. 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). (paper)