Amplitude variability or close frequencies in pulsating stars -: the δ Scuti star FG Vir

The nature of the observed amplitude variability of several modes in the delta Scuti star FG Vir is examined. This is made possible by the extensive photometry obtained during 2002-04, as well as the long photometric time-base starting in 1992. In this star, three frequencies show strong amplitude and phase variations. In the power spectrum, these frequencies also show up as frequency doublets. However, since true amplitude variability of a single frequency can also lead to (false) frequency doublets in the power spectrum, a specific test examining in detail the observed amplitude and the phase variations of an assumed single frequency is applied. For the frequencies at 12.15 and 23.40 cycle d(-1), it is shown that the amplitude variability of a single mode can be ruled out. In particular, an important property of beating between two modes is fulfilled: the amplitude and the phase vary synchronously with a phase shift close to 90 degrees. The origin of the amplitude variability of a third mode, namely, near 19.86 cycle d(-1), is not clear due to the long beat period of 20+ yr, for which the amplitude-phase test suffers from gaps in the coverage. However, even for this frequency, the amplitude variations can be expressed well by a mathematical two-mode model. If we examine these three close frequency pairs together with other (usually more widely separated) close frequencies in FG Vir, 18 pairs of frequencies with separations closer than 0.10 cycle d(-1) have been detected. It is shown that the majority of the pairs occur near the theoretically expected frequencies of radial modes. Mode identifications are available for only a few modes: the only detected radial mode at 12.15 cycle d(-1) is part of a close pair. It is shown that accidental agreements between the frequencies of excited modes can be ruled out because of the large number of detected close frequency doublets.