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Photometric monitoring during the period 1993–1997 has been carried out in the UBV system for the cool extreme helium stars BD+1°4381 (FQ Aqr) and BD−1°3438 (NO Ser). Periodograms of the data for both stars for each season of observation show the apparent presence of several periods but, if real, none seems to persist for more than one season. Analysis of the full data set for each star reveals no significant periodicities, although models predict that highly non-adiabatic pulsational variations should occur in hydrogen-deficient stars at these temperatures (̃ 10 000 K).
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Kilkenny et al. (1999) studied this question.
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