Accurate dynamical masses from binaries with extreme brightness ratios

High-resolution cross-correlation techniques are a powerful tool to explore exoplanet atmospheres. We introduce their application to binary stars from the EBLM project. In particular, we focus on eclipsing, high-contrast binaries consisting of a solar-type star and a low mass M-dwarf companion. These binary systems have typical brightness ratios ∼ 0.1% at optical wavelengths and appear as single-lined binaries in optical high-resolution spectra. Since the spectral features of low-mass stars are well known, we can recover the signal of the M-dwarf companion from what otherwise appears as noise, and accurately measure the masses of both stars, turning the system into a double-lined binary. We show that this application is a powerful new tool to not only derive accurate and precise dynamical masses for fully convective stars, but also to extract phase-dependent information. We will furthermore discuss the application to high-resolution infrared spectroscopy to characterise stars at the very bottom of the main-sequence.