Period determinations of spotted stars

The general behaviours of photometric, radial velocity and polarimetric observational investigations are explored in terms of the signals generated by a spot or patch in co-rotation with a star. It is demonstrated that the results of any period analysis of data depends on the nature of the chosen measurement diagnostic. Unlike photometry, analysis of radial velocity measurements is likely to provide considerable power at periods corresponding to the harmonics of the stellar rotational period. This conclusion may have bearing on Harmanecʹs "submultiple period scenario" in connection with the understanding of the behaviour of earlytype stars. Period analysis of polarimetric data is also likely to provide strong power in the harmonics of the fundamental rotational period. This analysis of the signals from the three basic observational diagnostics shows the dangers of simply using one measurement parameter alone to investigate stellar temporal behaviour and how interpretational confusion can arise when the results from different exercises are compared.