Light variations of massive stars (alpha Cygni variables). IX.

High-precision VBLUW photometry of 22 highly luminous OBA-type stars, 13 of which are Alpha Cygni variables and two of which are variable only in color, is presented. It is suggested that sigma (V-B) increases faster than sigma (B-L) with later spectral types due to the absence of stochastic metal line strength variations in the V band. It is assumed that short-time-scale stellar spot activity is present, or that temperature and density waves move through the atmospheres with the same time scales, producing these line strength variations independent of the continuum variations. The possibility of differentiating stars which lie close to each other in the HR diagram but have different evolutionary histories (and thus different metal abundances) with the aid of the maximum light amplitude/sigma (V-B) diagram is considered.