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We present recent measurements of the orbital motion in the binaries, DF Tauand ZZ Tau, and in the triples, Elias 12, T Tau, and V853 Oph. We observedthese systems with the Fine Guidance Sensors on the Hubble Space Telescope andwith adaptive optics imaging at the W. M. Keck and Gemini North Observatories.Based on our measurements and those presented in the literature, we performpreliminary orbital analyses for DF Tau, ZZ Tau, Elias 12 Na-Nb, and T TauSa-Sb. Because the orbital coverage in most of these systems does not yet spana sufficient portion of the orbit, we are not able to find definitive orbitsolutions. By using a Monte Carlo search technique, we explored the orbitalparameter space allowed by the current set of data available for each binary.We constructed weighted distributions for the total mass of the binariesderived from a large sample of possible orbits that fit the data. These massdistributions show that the total mass is already well-defined. We computetotal mass estimates of 0.78 (+0.25/-0.15) Msun, 0.66 (+0.15/-0.11) Msun, 1.13(+0.36/-0.09) Msun, and 4.13 (+1.58/-0.97) Msun for DF Tau, ZZ Tau, Elias 12Na-Nb, and T Tau Sa-Sb respectively, using a distance of 140 pc. For Elias 12Na-Nb, where the orbital coverage spans ~ 164 deg, we compute a preliminaryorbit solution with a period of ~ 9-12 years. By including an earlier lunaroccultation measurement, we also find a likely orbit solution for ZZ Tau, witha period of ~ 32 years. With additional measurements to continue mapping theorbits, the derived dynamical masses will be useful in constraining thetheoretical tracks of pre-main sequence evolution.Comment: 55 pages including 10 tables and 17 figures; accepted for publication  in A

Dynamical Mass Estimates for Incomplete Orbits: Young Multiple Stars in Taurus and Ophiuchus