The Absolute Parameters for NSVS11868841and the Oversized Stars in Low-Mass Eclipsing Binaries

Abstract

Spectroscopic observations of the low-mass eclipsing binary NSVS 11868841 have been obtained and the radial velocities were derived for both components. The masses and radii determined for the components are M1 = 0.870 ± 0.074 M⊙, M2 = 0.607±0.053 M⊙ and R1 = 0.983±0.030 R⊙, R2 = 0.901±0.026 R⊙. Both the primary and secondary stars’ radii are 10% and 57% larger than those of zeroage-main-sequence stars with the same masses. This discrepancy may arise from the large spot coverage of both stars. We collected absolute parameters of 21 low mass double-lined eclipsing binaries and compared their positions in the mass-radius and mass-effective temperature panels. The large radii and lower effective temperatures found for NSV 11868841 are solved neither with differences in metallicity nor in mixing length parameters. These discrepancies for low mass stars may be due to magnetic fields inhibiting the convective energy transport, which lead to a large magnetic spot coverage of the surface of such low mass stars.