  We study features in the optically detected magnetic resonance (ODMR) signals
associated with negatively charged nitrogen-vacancy (NV) centers coupled to
other paramagnetic impurities in diamond. Our results are important for
understanding ODMR line shapes and for optimization of devices based on NV
centers. We determine the origins of several side features to the unperturbed
NV magnetic resonance by studying their magnetic field and microwave power
dependences. Side resonances separated by around 130 MHz are due to hyperfine
coupling between NV centers and nearest-neighbor C-13 nuclear spins. Side
resonances separated by approximately {40, 260, 300} MHz are found to originate
from simultaneous spin flipping of NV centers and single substitutional
nitrogen atoms. All results are in agreement with the presented theoretical
calculations.
