An infrared study of Galactic OH/IR stars. III. Variability properties
  of the Arecibo sample
  We present the results of a near-infrared (NIR) monitoring program carried
out between 1999 and 2005 to determine the variability properties of the
Arecibo sample of OH/IR stars. The sample consists of 385 IRAS-selected
Asymptotic Giant Branch (AGB) candidates, for which their O-rich chemistry has
been proven by the detection of 1612 MHz OH maser emission. The monitoring data
was complemented by data collected from public optical and NIR surveys. We
fitted the light curves obtained in the optical and NIR bands with a model
using an asymmetric cosine function, and derived a period for 345 sources (90%
of the sample). Based on their variability properties, most of the Arecibo
sources are classified as long-period large-amplitude variable stars (LPLAV),
4% as (candidate) post-AGB stars, and 3% remain unclassified although they are
likely post-AGB stars or highly obscured AGB stars. The period distribution of
the LPLAVs peaks at 400 days, with periods between 300 and 800 days for most of
the sources, and has a long tail up to 2100 days. Typically, the amplitudes are
between 1 and 3 mag in the NIR and between 2 and 6 mag in the optical. We find
correlations between periods and amplitudes, with larger amplitudes associated
to longer periods, as well as between the period and the infrared colours, with
the longer periods linked to the redder sources. Among the post-AGB stars, the
light curve of IRAS 19566+3423 was exceptional, showing a large systematic
increase in K-band brightness over 7 years.
