A photometric study of AM Herculis in the normal and reversed soft X-ray modes

A long-term study of the optical behaviour of AM Her is presented, covering the period between 1983 September and 1985 March. During this period the system displayed the recently discovered reversed mode of soft X-ray behaviour, as well as the already much longer known normal mode. We compare the optical light curves in the two modes and find them remarkably similar. Systematic displacements of the primary minima with respect to the old published ephemeris have been detected. If the minimum offsets are caused by a continuous decrease of the rotational period of the white dwarf, they indicate a period change in a time-scale of 106yr. Some possible interpretation of this phenomenon is discussed.