2MASS J05162881+2607387: A New Low-mass Double-lined Eclipsing Binary

We show that the star known as 2MASS J05162881+2607387 (hereafter J0516) is a double-lined eclipsing binary with nearly identical low-mass components. The spectroscopic elements derived from 18 spectra obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope during the fall of 2005 are K1 = 88.45 ± 0.48 and K2 = 90.43 ± 0.60 km s-1, resulting in a mass ratio of q = K1/K2 = 0.978 ± 0.018 and minimum masses of M1 i = 0.775 ± 0.016 M☉ and M2 i = 0.759 ± 0.012 M☉, respectively. We have extensive differential photometry of J0516 obtained over several nights between 2004 January and March (epoch 1) and between 2004 October and 2005 January plus 2006 January (epoch 2) using the 1 m telescope at the Mount Laguna Observatory. The source was roughly 0.1 mag brighter in all three bandpasses during epoch 1 when compared to epoch 2. Also, phased light curves from epoch 1 show considerable out-of-eclipse variability, presumably due to bright spots on one or both stars. In contrast, the phased light curves from epoch 2 show little out-of-eclipse variability. The light curves from epoch 2 and the radial velocity curves were analyzed using our ELC code with updated model atmospheres for low-mass stars. We find the following: M1 = 0.787 ± 0.012 M☉, R1 = 0.788 ± 0.015 R☉, M2 = 0.770 ± 0.009 M☉, and R2 = 0.817 ± 0.010 R☉. The stars in J0516 have radii that are significantly larger than model predictions for their masses, similar to what is seen in a handful of other well-studied low-mass double-lined eclipsing binaries. We compiled all recent mass and radius determinations from low-mass binaries and determine an empirical mass-radius relation of the form R(R☉) = 0.0324 + 0.9343M(M☉) + 0.0374M2(M☉).