Magnetic structure in cool stars.

Ca II H and K fluxes were measured in 31 rapidly rotating main-sequence stars of type F and in seven single-line main-sequence spectroscopic binaries with periods less than 10 days. Most of the rapidly rotating stars observed appear to have relatively high fluxes compared to the slowly rotating stars of the same spectral type. Calculations show that the effect of rotational broadening is negligible and that the fluxes greater than normal measured in these stars must be a result of the increased Ca II H and K emission. All main-sequence binaries with periods less than nine days show eccentricities smaller than 0.1, which indicates that in these systems the rotational and orbital motions are probably synchronized.