UBVRI photometric monitoring of 7 rapidly rotating late-type dwarfs in the Alpha Persei cluster^*

¹ Laboratoire d'Astrophysique, Observatoire de Grenoble, Université Joseph Fourier, B.P. 53X, 38041 Grenoble Cedex, France e-mail: bouvier@gag.observ-gr.fr
² Canada-France-Hawaii Telescope Corporation, P.O. Box 1597, Kamuela, HI 96743, U.S.A. e-mail: bouvier@cfht.hawaii.edu

Received: 7 February 1996
Accepted: 15 April 1996

Abstract

The UBVRI light curves of 7 late-type dwarfs amongst the fastest rotators of the Alpha Per cluster (vsini ≥140 km s^-1) are presented. The shape of the light curves suggests that the photometric variations are most often dominated by a single group of cool spots located at intermediate or high latitude on the stellar surface. Assuming that starspots are good tracers of the stellar magnetic field, the smooth light curves indicate the existence of a large scale, slowly varying magnetic structure at the surface of these ultrafast rotators.