* Astronomy

Title: Physical Properties of the Low-Mass Eclipsing Binary NSVS 02502726
Authors: Jae Woo Lee, Jae-Hyuck Youn, Seung-Lee Kim, Chung-Uk Lee

NSVS 02502726 has been known as a double-lined, detached eclipsing binary that consists of two low-mass stars. We obtained BVRI photometric follow-up observations in 2009 and 2011 to measure improved physical properties of the binary star. Each set of light curves, including the 2008 data given by Cakirli et al., was simultaneously analysed with the previously published radial-velocity curves using the Wilson-Devinney binary code. The conspicuous seasonal light variations of the system are satisfactorily modelled by a two-spot model with one starspot on each component and by changes of the spot parameters with time. Based on 23 eclipse timings calculated from the synthetic model and one ephemeris epoch, an orbital period study of NSVS 02502726 reveals that the period has experienced a continuous decrease of -5.9 x 10^{-7} d yr^{-1} or a sinusoidal variation with a period and semi-amplitude of 2.51 yrs and 0.0011 d, respectively. The timing variations could be interpreted as either the light-travel-time effect due to the presence of an unseen third body, or as the combination of this effect and angular momentum loss via magnetic stellar wind braking. Individual masses and radii of both components are determined to be M_1=0.689 ±0.016 solar masses, M_2=0.341 ±0.009 solar masses, R_1=0.707 ±0.007 solar radii, and R_2=0.657 ±0.008 solar radii. The results are very different from those of Cakirli et al. with the primary's radius (0.674 ±0.006 solar radii) smaller the secondary's (0.763 ±0.007 solar radii). We compared the physical parameters presented in this paper with current low-mass stellar models and found that the measured values of the primary star are best fitted to a 79-Myr isochrone. The primary is in good agreement with the empirical mass-radius relation from low-mass binaries, but the secondary is oversized by about 85%.

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