Evolutionary PMS tracks for 0.1 to 0.4 solar masses and solar metallicity, shown as a function of effective temperature and absolute K-magnitude (Baraffe et al. 1998). Isochrones are drawn for 10 (black), 50, 100, and 150 Myr (green). The components of Rst 137B are indicated by error bars according to our spectroscopically determined T_eff and the photometry of Janson et al. (2007), see Sect. 5.1 for details.