Asteroseismology of the β Cephei star ν Eridani -: I.: Photometric observations and pulsational frequency analysis

We undertook a multisite photometric campaign for the beta Cephei star nu Eridani. More than 600 h of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights. The frequency analysis of our measurements shows that the variability of nu Eri can be decomposed into 23 sinusoidal components, eight of which correspond to independent pulsation frequencies between 5 and 8 cd(-1). Some of these are arranged in multiplets, which suggests rotational m-mode splitting of non-radial pulsation modes as the cause. If so, the rotation period of the star must be between 30 and 60 d. One of the signals in the light curves of nu Eri has a very low frequency of 0.432 cd(-1). It can be a high-order combination frequency or, more likely, an independent pulsation mode. In the latter case, nu Eri would be both a beta Cephei star and a slowly pulsating B (SPB) star. The photometric amplitudes of the individual pulsation modes of nu Eri appear to have increased by about 20 per cent over the last 40 years. So have the amplitudes of the dominant combination frequencies of the star. Among the latter, we could only identify sum frequencies with certainty, not difference frequencies, which suggests that neither light-curve distortion in its simplest form nor resonant mode coupling is their single cause. One of our comparison stars, mu Eridani, turned out to be variable with a dominant time-scale of 1.62 d. We believe either that it is an SPB star just leaving its instability strip or that its variations are of rotational origin.