The Nearby Low-Mass Visual Binary Wolf 424

We present new measurements of the relative positions of the components of the low-mass visual binary Wolf 424 (M5.5 Ve) made with the Fine Guidance Sensors on the Hubble Space Telescope. Previous analyses of the astrometric orbit of this system indicated that the components have substellar masses (Heintz; Schultz et al.), raising considerable interest as the first candidate brown dwarfs to have their masses measured dynamically. These studies relied partly on visual and photographic measurements, which are affected by systematic errors in the angular separation and have thus biased those solutions. Our new orbital solution using only the position angles of the early measurements together with all available modern high-resolution observations including our own shows that the component masses are clearly above the substellar limit (M_A=0.143+/-0.011 M_solar, M_B=0.131+/-0.010 M_solar), and thus they are not brown dwarfs. Recent evolutionary models for low-mass stars show good agreement with the location of Wolf 424A and B in the mass-luminosity diagram in the K band. In addition, we show that while the secondary appears to have normal colors compared to similar M dwarfs, the primary star is fainter than expected at optical wavelengths (B, V), possibly owing to significant spot coverage. This is consistent with the strong flaring activity displayed by the system.