  RX J0513.1+0851 and RX J0539.9+0956 were previously identified as young,
low-mass, single-lined spectroscopic binary systems and classified as
weak-lined T Tauri stars at visible wavelengths. Here we present radial
velocities, spectral types, vsini values, and flux ratios for the components in
these systems resulting from two-dimensional cross-correlation analysis. These
results are based on high-resolution, near-infrared spectroscopy taken with the
Keck II telescope to provide a first characterization of these systems as
double-lined rather than single-lined. It applies the power of infrared
spectroscopy to the detection of cool secondaries; the flux scales as a less
steep function of mass in the infrared than in the visible, thus enabling an
identification of low-mass secondaries. We found that the RX J0513.1+0851 and
RX J0539.9+0956 primary stars are fast rotators, 60 km/s and 80 km/s
respectively; this introduces extra difficulty in the detection of the
secondary component as a result of the quite broad absorption lines. To date,
these are the highest rotational velocities measured for a pre-main sequence
spectroscopic binary. The orbital parameters and mass ratios were determined by
combining new visible light spectroscopy with our infrared data for both
systems. For RX J0513.1+0851, we derived a period of ~4 days and a mass ratio
of q = 0.46 +/- 0.01 and for RX J0539.9+0956, a period of ~1117 days and a mass
ratio of q = 0.66 +/- 0.01. Based on our derived properties for the stellar
components, we estimate the luminosities and hence distances to these binaries
at 220 pc and 90 pc. They appear to be significantly closer than previously
estimated.
