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We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensedz ~ 1.9 galaxies, A2 and S2, located behind the cluster AC 114. Thanks to largemagnification factors, we have been successful in detecting rest-frame opticalemission lines in star-forming galaxies 1 to 2 magnitudes fainter than inprevious studies of Lyman break galaxies (LBGs) at z ~ 3. From the Haluminosity, we estimate star formation rates (SFRs) which are 7 to 15 timeshigher than those inferred from the UV continuum flux at 1500 ang without dustextinction correction. The behavior of S2 and A2 in terms of O/H and N/Oabundance ratios are very different, and they are also different from typicalLBGs at z ~ 3. S2 is a low-metallicity object (Z ~ 0.03 Zsun) with a low N/Oratio, similar to those derived in the most metal-poor nearby HII galaxies. Incontrast, A2 is a high-metallicity galaxy (Z ~ 1.3 Zsun) with a high N/Oabundance ratio, similar to those derived in the most metal-rich starburstnucleus galaxies. The virial masses, derived from emission-line widths, are 0.5and 2.4 x 10^10 Msun, for S2 and A2 respectively. Thanks to the gravitationalamplification, the line profiles of S2 are spatially resolved, leading to avelocity gradient of +- 240 km/s, which yields a dynamical mass of ~ 1.3 x10^10 Msun within the inner 1 kpc radius. Combining these new data with thesample of LBGs at z ~ 3, we conclude that these three galaxies exhibitdifferent physical properties in terms of SFRs, abundance and mass-to-lightratios, and reddening. High-redshift galaxies of different luminosities couldthus have quite different star formation histories (abridged version).Comment: 11 pages, 8 figures. Accepted for publication in A&

Physical properties of two low-luminosity $\mathsf{{\vec z} \sim 1.9}$ galaxies behind the lensing cluster AC 114