Unusual variability among early F-type stars: HD 224638 and HD 224945

The two main-sequence early F-type stars HD 224638 and HD 224945 show light variations of an uncommon kind. We obtained about 1000 datapoints for each star on 20 nights over a period of 35 d (corresponding to 118 h of monitoring). Frequency analysis by several different techniques shows that both light curves contain two close periodic terms of about 0.8 d–1. The two terms of HD 224638 have a double-wave shape, while those of HD 224945 have a triple-wave shape. However, these terms are unable fully to account for the light variations. A similar behaviour could also be explained by single pseudo-periodic variations with a characteristic frequency of about 0.8 d–1. High-resolution spectrograms give evidence of line profile variations in HD 224638. A discussion of the characteristics of these stars, together with those of the few similar objects found in the literature, shows that they constitute a very homogeneous class whose nature is, however, still unclear. The most promising mechanisms that can explain this phenomenon are spot activity and non-radial g-mode pulsations. Both these possibilities have many consequences in the context of stellar structure and evolution.