Ultraviolet spectral variations of FK Comae and V 1794 Cygni

•We analyze spectral time variations in two stars, FK Comae and V1794 Cygni.•The emission lines fluxes for FK Com and V1794 Cyg vary by factors of ∼2–3.•The fluxes of the C II, C IV, and He II lines of FK Com are comparable to V1794 Cyg.•The variations are consistent with some models that have been previously proposed.

Ultraviolet spectra of FK Comae and V1794 Cygni observed with the Hubble Space Telescope Cosmic Origins Spectrograph (HST COS) and the International Ultraviolet Explorer (IUE) satellites were analyzed for the period 1981–2011. Temporal variations of line fluxes of the O I 1306 Å, C II 1336 Å, C IV 1550 Å, He II 1640 Å and Mg II k & h 2800 Å, produced in the transition regions and chromospheres of these stars, imply variations in density and temperature changes in the line emitting regions as a result of the rapid rotation and magnetic fields responsible for stellar activity. Results are consistent with the models of Ramsey et al. (1981), Oliveira and Foing (1999), and Korhonen et al. (2000).