We present a spectroscopic study of candidate brown dwarf members of the Orion Nebula cluster (ONC ). We obtained new J-and/or K-band spectra of $100 objects within the ONC that are expected to be substellar on the basis of their K magnitudes and HÀK colors. Spectral classification in the near-infrared of young low-mass objects is described, including the effects of surface gravity, veiling due to circumstellar material, and reddening. From our derived spectral types and existing near-infrared photometry, we construct an H-R diagram for the cluster. Masses are inferred for each object and used to derive the brown dwarf fraction and assess the mass function for the inner 5A1 ; 5A1 of the ONC, down to $0.02 M . The logarithmic mass function rises to a peak at $0.2 M , similar to previous initial mass function determinations derived from purely photometric methods but falls off more sharply at the hydrogen-burning limit before leveling through the substellar regime. We compare the mass function derived here for the inner ONC with those presented in recent literature for the sparsely populated Taurus cloud members and the rich cluster IC 348. We find good agreement between the shapes and peak values of the ONC and IC 348 mass distributions but little similarity between the ONC and Taurus results.

The h and χ Per "double cluster" is examined using wide-field (0.98 • × 0.98 • ) CCD UBV imaging supplemented by optical spectra of several hundred of the brightest stars. Restricting our analysis to near the cluster nuclei, we find identical reddenings (E(B − V ) = 0.56 ± 0.01), distance moduli (11.85 ± 0.05), and ages (12.8 ± 1.0 Myr) for the two clusters. In addition, we find an IMF slope for each of the cluster nuclei that is quite normal for high-mass stars, Γ = −1.3±0.2, indistinguishable from a Salpeter value. We derive masses of 3700 M ⊙ (h) and 2800 M ⊙ (χ) integrating the PDMF from 1 to 120 M ⊙ . There is evidence of mild mass segregation within the cluster cores. Our data are consistent with the stars having formed at a single epoch; claims to the contrary are very likely due to the inclusion of the substantial population of early-type stars located at similar distances in the Perseus spiral arm, in addition to contamination by G and K giants at various distances. We discuss the uniqueness of the double cluster, citing other examples of such structures in the literature, but concluding that the nearly identical nature of the two cluster cores is unusual. We fail to settle the long-standing controversy regarding whether or not the double cluster is the