5 Colour variations

As a first indication of the nature of the photometric variations, it is clearly useful to obtain some insight into P Cygni's colour variations. From a visual inspection of the light and colour curves one obtains the impression that B-Vincreases (i.e. the star reddens) when V decreases (i.e. when the star brightens). To check this behaviour more objectively, we proceed as follows: Treating data from each observing season (which happens to coincide with the calendar year) separately, we calculate the average value of B-V in each $0\hbox{$.\!\!^{\rm m}$ }02$-wide bin of V (i.e. $4.900 > V \geq 4.880$; $4.880 > V \geq 4.860$, etc.), plot the mean value of the V measurements in each bin against the average B-V values, fit a straight line using a simplified least-squares method, and determine the slope dV/d(B-V) of the resulting straight line for each year. Every year shows a slope dV/d(B-V) between -5 and -11, implying that a decrease in V, i.e. when the star brightens, produces an increase in (B-V), i.e. the star reddens. Bearing in mind that this refers to values of V and (B-V) over a year, we conclude that the above-mentioned variations with time scales between 60 and 130 days are best identified with the so-called 100 d-type micro-variations often found in S Dor variables near maximum brightness (van Genderen et al. 1997a,b).

Because the amplitude of the 17-day variations is smaller, the resultant smaller variations in (B-V) and the dispersion in the V versus (B-V) relation make the results of an analysis along the above lines more unreliable. Visual inspection of the detailed light and colour curve shows that the colour behaviour in the maxima and minima differs from extremum to extremum: sometimes it is a local maximum (i.e. redder), sometimes it is a local minimum (i.e. bluer), and sometimes it is neither. This is not entirely similar to the colour behaviour of the $\alpha$ Cygni-type variations in other stars (e.g. van Genderen et al. 1997a,b), where the colour is always bluer in the light maxima.