Spectroscopic study of the variability of three northern Of supergiants

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 time-scales from a few hours up to a few years. Time Variance Spectrum (TVS) and Fourier analyses were performed in order to characterize their spectral variability. We report on a correlated significant line profile variability in the prominent He ii λ 4686 and Hβ lines most likely related to the strong stellar winds. The variability pattern is similar for the three stars investigated (HD14947, HD15570 and HD16691), and the main differences are more quantitative than qualitative. However, the reported time-scales are somewhat different, and the most striking variability pattern is reported for HD16691. We did not find any clear evidence for binarity, and we focus mainly on an interpretation based on a single star scenario. We show that the behaviour of the three stars investigated in this study present strong similarities, pointing to a putative common scenario, even though a few differences should be noted. Our preferred interpretation scheme is that of Large Scale Corotating Structures modulating the profile of the lines that are produced in the strong stellar wind. Subject headings: stars: early-type – stars: mass-loss – stars: supergiants – stars: individual: HD 14947 – stars: individual: HD 15570 – stars: individual: HD 16691