  We re-examined photometry (VBLUW, UBV, uvby) of the yellow hypergiant HR
5171A made a few decades ago. In that study no proper explanation could be
given for the enigmatic brightness excesses in the L band (VBLUW system,
lambda_eff=3838 A). In the present paper, we suggest that this might have been
caused by blue luminescence (BL), an emission feature of neutral polycyclic
aromatic hydrocarbon molecules (PAHs), discovered in 2004. It is a fact that
the highest emission peaks of the BL lie in the L band. Our goals were to
investigate other possible causes, and to derive the fluxes of the emission. We
used two-colour diagrams based on atmosphere models, spectral energy
distributions, and different extinctions and extinction laws, depending on the
location of the supposed BL source: either in Gum48d on the background or in
the envelope of HR 5171A. False L-excess sources, such as a hot companion, a
nearby star, or some instrumental effect, could be excluded. Also, emission
features from a hot chromosphere are not plausible. The fluxes of the L excess,
recorded in the data sets of 1971, 1973, and 1977 varied (all in units of
10^(-10) W m^(-2) micron^(-1)) between 1.4 to 21, depending on the location of
the source. A flux near the low side of this range is preferred. Small
brightness excesses in uv (uvby system) were present in 1979, but its
connection with BL is doubtful. For the L fluxes we consider the lowest values
as more realistic. The uncertainties are 20-30 %. Similar to other yellow
hypergiants, HR 5171A showed powerful brightness outbursts, particularly in the
1970s. A release of stored H-ionization energy by atmospheric instabilities
could create BL emitted by neutral PAHs.
