2006A&A...456..253M

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST), we obtain spectra from 20 spatially-resolved, ultracool dwarfs, eighteen of which belong to nine very low-mass binary systems with known angular separations in the range 0.37"-0.098". For these dwarfs, we derive their spectral types to be in the range dM7.5 to dL6 by either obtaining their PC3 index value or by comparing our STIS spectra with ground-based spectra of similar spectral resolution from Martin et al. (1999AJ....118.2466M). We have searched for H_α emission in each object, but the emission is clearly detected in only two of them. We find that the distribution of H_α emission in our sample is statistically different from that of single field dwarfs, suggesting an intriguing anti-correlation between chromospheric activity and binarity for M7-M9.5 dwarfs. We provide strength measuments of the main photospheric features. We derive calibrations of spectral subclasses versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5 binaries that have known trigonometric parallaxes.