Near-IR variability properties of a selected sample of AGB stars

We present the results of a near-infrared monitoring programme of a selected sample of stars, initially suspected to be Mira variables and OH/IR stars, covering more than a decade of observations. The objects monitored cover the typical range of IRAS colours shown by O-rich stars on the asymptotic giant branch and show a surprisingly large diversity of variability properties. Sixteen objects are confirmed as large-amplitude variables. Periods between 360 and 1800(d) and typical amplitudes 1(m) less than or similar to Delta K less than or similar to 2(m) could be determined for nine of them. In three light curves, we find a systematic decrease in the mean brightness, and two light curves show pronounced asymmetry. One source, IRAS 07222 - 2005, shows infrared colours typical of Mira variables, but it pulsates with a much longer period (approximate to 1200(d)) than a normal Mira. Two objects are either close to ( IRAS 03293+6010) or probably in ( IRAS 18299 - 1705) the post-AGB phase. In IRAS 16029 - 3041 we found a systematic increase in the H - K colour of approximate to 1(m), which we interpret as evidence of a recent episode of enhanced mass loss. IRAS 18576+0341, a heavily obscured luminous blue variable was also monitored. The star showed a continued decrease in brightness over a period of 7 years ( 1995 - 2002).