  We present rest-frame optical spectra for a sample of 9 low-mass star-forming
galaxies in the redshift range 1.5 < z < 3 which are gravitationally lensed by
foreground clusters. We used Triplespec, an echelle spectrograph at the Palomar
200-inch telescope that is very effective for this purpose as it samples the
entire near-infrared spectrum simultaneously. By measuring the flux of nebular
emission lines we derive gas phase metallicities and star formation rates, and
by fitting the optical to infrared spectral energy distributions we obtain
stellar masses. Taking advantage of the high magnification due to strong
lensing we are able to probe the physical properties of galaxies with stellar
masses in the range 7.8 < log M/Msun < 9.4 whose star formation rates are
similar to those of typical star-forming galaxies in the local universe. We
compare our results with the locally determined relation between stellar mass,
gas metallicity and star formation rate. Our data are in excellent agreement
with this relation, with an average offset <Delta log O/H> = 0.01 +/- 0.08,
suggesting a universal relationship. Remarkably, the scatter around the
fundamental metallicity relation is only 0.24 dex, smaller than that observed
locally at the same stellar masses, which may provide an important additional
constraint for galaxy evolution models.
