tokens: the course of the observations (typically 33 d for Campaign 0). Eclipsing binaries and ellipsoidal variables of high amplitude are avoided. If we consider only stars where the harmonic is visible, then 86 out of 265 (32 per cent) 09–B9 stars are rotational variables. In addition, if we add stars with just a single isolated peak (no harmonic), then 142 out of 265 stars (54 per cent) are rotational variables. Balona et al. ( 2015 ) has already shown that rotational modulation is probably present in many late B stars in the Kepler field. This study shows that this effect is likely to be present in early B stars as well. Indeed, the large fraction of B-type rotational variables in the K2 data is probably an underestimate because these data are less precise and of much shorter duration than the Kepler data. The only test that is available for rotational modulation is to compare the rotational velocity obtained from the photometric period (using an appropriate stellar radius) with the rotational velocity derived from spectroscopic line broadening measurements. This could be done successfully for the A stars (Balona 2013 ) because of the large number (875) of stars with known periods and radii derived from multicolour CCD photometry. Unfortunately, there are too few B stars in our sample to use this statistical test. Moreover, stellar radii cannot be estimated with sufficient precision. There are only seven stars in Table 2 with known values of the projected rotational velocity. Rotational modulation implies some kind of surface brightness inhomogeneity. The nature of such a ‘spot’ cannot be determined at present. However, it is difficult to conceive of starspot formation without the presence of a magnetic field. We therefore suggest that magnetic fields are probably present in stars with radiative envelopes. According to Cantiello Braithwaite ( 2011 ), dynamo action in a subsurface convection zone in such stars might generate a magnetic field. In this case magnetic spots of size comparable to the local pressure scaleheight can manifest themselves as hot, bright spots. A theory of dynamo action in radiative envelopes which involves differential rotation has been developed by Spruit ( 1999 , 2002 ) and Maeder Meynet ( 2004 ). In this theory, a magnetic instability in the toroidal field wound up by differential rotation replaces the role of convection in closing the field amplification loop in conventional dynamo theory. Zahn, Brun Mathis ( 2007 ) has verified that an instability is generated, but concludes that a dynamo cannot be sustained by this effect. Arlt Rüdiger ( 2011 ) found that feedback by the Lorentz force diminishes differential rotation and brings field growth to a halt in a few thousand years for intermediate mass stars. It seems that we still do not have an acceptable explanation for the generation of a magnetic field in radiative envelopes. One way to circumvent the generation of magnetic fields in radiative envelopes is to suppose
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