R110 = HDE 269662 was discovered as an LBV by Stahl et al. (1990), but was suspected of photometric variability already in 1984 (Stahl et al. 1984). Table 2 (click here) lists the global characteristics of light and colour curves during the last 40y.
Figure 12 (click here) shows the recent photometric history of R110 from
the first photoelectric observations around 1957 up to 1994, in various
photometric systems. All scattered observations are represented by dots
in the main diagram for 
and in the panel for 
. All
horizontal line pieces represent the observations of Walraven &
Walraven (1977) made in the interval 1957-1963 (only one average is
given by them). We formed seasonal or monthly averages of the 
magnitudes obtained by the LTPV group. These are
represented by the circles in the insert below the main light curve,
showing the light curve 1989-1994 in more detail. Bracketed circles
are based on one or two nights only. The standard deviations amount to
, while 001 is normal. Thus, micro variations are
present, but a period analysis is impossible because of a lack of
sufficient observations.
The dots in the insert represent average magnitudes
transformed from unpublished Walraven V values, each comprising
one month of observations. Their standard deviations amount to
, partly caused by the micro variations (see above).
The agreement between both magnitude sequences is excellent.
The smooth curve sketched in the insert is also sketched in
the main diagram for the light curve.
The averages of the Strömgren colours from the LTPV group are
represented in Fig. 12 (click here) by dots connected by lines. Their standard
deviations amount to 001 for b-y, 002 for v-b and 004 for u-v.
Note that their scales as well as that of 
are twice the
scale for . The average Walraven colours (in the natural
system) are also plotted as dots and connected by lines; these
colours are also given in magnitude scale.
Therefore, the scales are given as (V-B), etc.
Standard deviations amount to 001 for V-B and B-L, 002 for B-U
and 008 for U-W.
A broken line connects the Walraven colours obtained before 1963
with those obtained after 1989 across an interval for which no
observations are available. Evidently, the
evolution of the colours is dramatic and indicates an ongoing SD
phase (amplitude in 
), apparently
consisting of a succession of at least two SD cycles with a time
scale of 
. The more recent cycle also shows humps on
a time scale of 
(see insert in Fig. 12 (click here)).
The HDE magnitude determined at the end of the previous
century has been corrected to by Feast et al. (1960):
111, see arrow in the upper panel of Fig. 12 (click here). This suggests that
the total range of the activity of R110 over this century must be at
least 14. Around 1960 the brightness was higher ( 
);
thus, this was no minimum state. Its corresponding spectral type
was B9I (Feast et al. 1960). By early 1989 (
) the spectral type had changed to F0Ia with a temperature of 
(Stahl et al. 1990). Wolf (1992) quoted
Zickgraf (private communication to Wolf) that in November 1991 (
) the colour suggested a spectral type as late as early G,
suggesting a temperature 
.
The deep reddening from 1989 to 1992 (Fig. 12 (click here)) should
not be a surprise: while rises by at most 01, in B, v, L
and U the star fades. These and other colour characteristics are similar
to, e.g., those of S Dor and R127 when they approached their maximum
visual brightness. However, they are more extreme in R110, which
lends support to the idea that the SD phenomenon is
not only confined to blue supergiants, giving rise to LBVs, but also to
yellow supergiants/hypergiants, the LYVs (de Jager & van Genderen
1989; van Genderen 1991; Stahl et al.
1990, see also Gallagher 1992).
For hot S Dor-type variables the SD phenomenon seems to be due mainly to a change in the stellar radius and only partly to a wind phenomenon. For the cooler variables the wind characteristics and the pseudo-photosphere (e.g. Stahl et al. 1990) may play an increasing role in the photometric behaviour. Whether R110 can be classified as an LYV cannot be answered before a reliable temperature determination during visual maximum has been made.