An upper bound on the 1.6 micron flux ratio of the companion to ρ coronae borealis

We use high-resolution infrared spectroscopy to investigate the 2001 report by Gatewood and colleagues that ρ CrB's candidate extrasolar-planet companion is really a low-mass star with mass 0.14 ± 0.05 M⊙. We do not detect evidence of such a companion; the upper bounds on the (companion-to-primary) flux ratio at 1.6 μm are less than 0.0024 and 0.005 at the 90% and 99% confidence levels, respectively. Using the H-band mass-luminosity relationship calculated by Baraffe and colleagues, the corresponding upper limits on the companion mass are 0.11 and 0.15 M⊙. Our results indicate that the infrared spectroscopic technique can detect companions in binaries with flux ratios as low as 0.01 to 0.02.