  We report the discovery of a new low-mass spectroscopic (SB2) stellar binary
system in the star-forming region of Upper Scorpius. This object, UScoCTIO5,
was discovered by Ardila (2000), who assigned it a spectral class of M4. A
KeckI HIRES spectrum revealed it to be double-lined, and we then carried out a
program at several observatories to determine its orbit. The orbital period is
34 days, and the eccentricity is nearly 0.3. The importance of such a discovery
is that it can be used to help calibrate evolutionary models at low masses and
young ages. This is one of the outstanding problems in the study of formation
mechanisms and initial mass functions at low masses. The orbit allows us to
place a lower limit of 0.64 +- 0.02 M_sol on the total system mass. The
components appear to be of almost equal mass. We are able to show that this
mass is significantly higher than predicted by evolutionary models for an
object of this luminosity and age, in agreement with other recent results. More
precise determination of the temperature and surface gravity of the components
would be helpful in further solidifying this conclusion.
