We thank the referee D. Pollacco for his valuable suggestions. The ISOCAM data presented in this paper was analysed using `CIA', a joint development by the ESA Astrophysics Division and the ISOCAM Consortium. The ISOCAM Consortium is led by the ISOCAM PI, C. Cesarsky, Direction des Sciences de la Matière, C.E.A., France. In this research, we have used, and acknowledge with thanks, data from the AAVSO International Database, based on observations submitted to the AAVSO by variable star observers worldwide. We also acknowledge the use of the archive of the CADC. This work is supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF), project P-11675.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999