The fact that the variability is periodic offers a major condition for the detection of such very small amplitude oscillations, since the Fourier analysis may be used in that case. Such a variable star would not have been detected on the basis of its standard deviation exclusively. Indeed, the standard deviation of the data used in Fig. 3 is = 0.0047, while the standard deviation around the fitted curve (the accuracy of the measurements) is = 0.0043, in perfect agreement with the knowledge of the properties of the Geneva photometric system. The difference = 0.0004 is very small and the star would have been considered as constant merely on the basis of the value of .
The study of long-period small-amplitude variable stars requires stable and accurate photometric instrumentation. In the case of HR 1960, these conditions were particularly required because of the length of the period, about one year. On the other hand, the discovery of such small semi-amplitude variation, 3 millimags, was possible thanks to its periodic character.
Finally, note that HR 1960 is very probably the long-period variable star with the smallest amplitude yet known.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999