Analysis of Kepler B stars: rotational modulation and Maia variables

We examine 4 yr almost continuous Kepler photometry of 115 B stars. We find that the light curves of 39 per cent of these stars can be described by a low-frequency sinusoid and its harmonic, usually with variable amplitude, which we interpret as rotational modulation. A large fraction (32 stars or 28 per cent) might be classified as ellipsoidal variables, but a statistical argument suggests that these are probably rotational variables as well. Ten stars (8 per cent of the total) have a peculiar periodogram feature which is common among A stars. The physical cause of this feature is unknown but very likely related to rotation. The large number of rotating variables suggests that star-spots are common in B stars and indicates that magnetic fields can be generated in radiative stellar envelopes. We find five stars (4 per cent) to be beta Cep (BECP) variables, all of which have low frequencies with relatively large amplitudes. The presence of low frequencies in BECP stars is an unsolved problem. There are 21 stars (18 per cent) with multiple low frequencies which could be described as slowly pulsating B (SPB) variables. Other pulsating stars do not fit into the general definition of BECP or SPB variables in that they are cool but pulsate with high frequencies. We assume that these are further examples of the anomalous pulsating stars which in the past have been called 'Maia' variables. There are 11 stars (10 per cent) of this type. We also examined 300 B stars observed in the K2 Campaign 0 field. We find 12 BECP/Maia candidates and 15 SPB variables.