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The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giantplanets and brown dwarfs around a sample of ~30 M-dwarfs. We have discoveredseveral low-mass companions by measuring the motion of our target starsrelative to their reference frames. The highest mass discovery thus far is G78-28B, a companion to the M-dwarf G 78-28A. The orbital period is 4.18 +/-0.03 y, the system mass is 0.565 +/- 0.055 Msolar, and the semi-major axis is2.19 +/- 0.10 AU. Imaging observations with the Keck laser guide star adaptiveoptics (LGSAO) and the Palomar AO instruments resolved the system and alsoyielded JHK-band delta magnitudes. We use the orbital solution, light ratios,and mass-luminosity relationships to derive component masses of MA = 0.370 +/-0.034 Msolar and MB = 0.195 +/- 0.021 Msolar. G 78-28B is of type M4 V basedupon its colors and mass. We also discovered GJ 231.1C, a companion to GJ231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but theorbital period is longer than our observing baseline; thus the systemparameters are less constrained. In GJ 231.1BC the masses are MB = 0.25 +/-0.06 Msolar and MC =0.12 +/- 0.02 Msolar. The inferred spectral type of GJ231.1C is M5 V. We demonstrate the results of the current state of massestimation techniques with our data.Comment: 25 pages, 8 figures, accepted for Ap

Masses of Astrometrically Discovered and Imaged Binaries: G78‐28AB and GJ 231.1BC