We have analysed three-colour photometry of 34 pulsating red giants that was published by Percy, Wilson and Henry (2001). As is common with semiregulars, many of these stars have long secondary periods (LSPs) of several years, in addition to the usual shorter period pulsations. The origin of LSPs is unclear, and may be due to binarity (Wood et al., 1999) or to strange pulsation modes casued by convection-pulsation interaction (Wood 2000). We measured pulsation amplitudes in VRI and found that the V/R and V/I amplitude ratios in most stars are systematically lower for LSPs than for the shorter periods. These results must be taken into account by any theory that attempts to explain LSPs.

 
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