  An increasing number of AGNs exhibit broad, double-peaked Balmer emission
lines, which are thought to arise from the outer regions of the accretion disk
which fuels the AGN. The line profiles are observed to vary on a characteristic
timescales of 5-10 years. The variability is not a reverberation effect; it is
a manifestation of physical changes in the disk. Our group has monitored a set
of 20 double-peaked emitters for the past 8 years (longer for some objects).
Here, we characterize the variability of the double-peaked H alpha line
profiles in five objects from our sample. By experimenting with simple models,
we find that disks with a single precessing spiral arm are able to reproduce
many of the variability trends that are seen in the data.
