The transition from early Of stars to WN-type objects is poorly understood. O-type supergiants with emission lines (OIf^+^) are considered to be intermediate between these two classes. The scope of this paper is to investigate the spectral variability of three Of^+^ supergiants. We constituted spectral time series of unprecedented quality for our targets (~200 spectra in total), essentially in the blue domain, covering timescales from a few hours up to a few years. Temporal Variance Spectrum and Fourier analyses were performed in order to characterize their spectral variability.

Cone search capability for table J/ApJ/704/964/table3 (Stellar parameters of HD 14947, HD 15570, and HD 16691, and estimated minimum and maximum rotation periods)