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To better understand the LBV phenomenon, we analyze multi-epoch andmulti-wavelength spectra and photometry of R71. Pre-outburst spectra areanalyzed with the radiative transfer code CMFGEN to determine the star'sfundamental stellar parameters. During quiescence, R71 has an effectivetemperature of $T_\mathrm{{eff}} = 15\,500~K$ and a luminosity oflog$(L_*/L_{\odot})$ = 5.78 and is thus a classical LBV, but at the lowerluminosity end of this group. We determine its mass-loss rate to $4.0 \times10^{-6}~M_{\odot}~$yr$^{-1}$. We present R71's spectral energy distributionfrom the near-ultraviolet to the mid-infrared during its present outburst.Mid-infrared observations suggest that we are witnessing dust formation andgrain evolution. Semi-regular oscillatory variability in the star's light curveis observed during the current outburst. Absorption lines develop a second bluecomponent on a timescale twice that length. The variability may consist of one(quasi-)periodic component with P ~ 425/850 d with additional variationssuperimposed. During its current S Doradus outburst, R71 occupies a region inthe HR diagram at the high-luminosity extension of the Cepheid instabilitystrip and exhibits similar irregular variations as RV Tau variables. LBVs donot pass the Cepheid instability strip because of core evolution, but theydevelop comparable cool, low-mass, extended atmospheres in which convectiveinstabilities may occur. As in the case of RV Tau variables, the occurrence ofdouble absorption lines with an apparent regular cycle may be due to shockswithin the atmosphere and period doubling may explain the factor of two in thelengths of the photometric and spectroscopic cycles.

Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71